OutlookGoogleSync and the other ways to keep two calendars aligned

Searching for outlookgooglesync usually means one of two things has already happened. Either a work calendar lives in Outlook and a personal calendar lives in Google, and neither one can be given up. Or a team moved between the two systems and the meetings did not all move with it. The search term itself is a clue about the expected answer: a piece of software sitting between the two accounts, copying events back and forth on a timer.

That software exists, it works, and there is a good chance it cannot run on the machine in front of you. Knowing why takes five minutes and saves a long afternoon.

The name points at two different tools

Two separate things get called outlookgooglesync, and they solve different problems.

The first is Outlook Google Calendar Sync, usually shortened to OGCS. It is an open source application maintained by an individual developer, published on GitHub, and free to download. It runs on the computer, signs in to both an Outlook calendar and a Google calendar, and copies events between them on a schedule that the user sets.

The second is Google Workspace Sync for Microsoft Outlook, shortened to GWSMO. This one is Google's own, and its purpose is narrower. It lets a Microsoft Outlook client use a Google Workspace account as its back end, so mail, calendar and contacts live with Google while the familiar Outlook window stays in place. It is aimed at organisations that moved to Google Workspace and did not want to retrain everyone.

The distinction matters when choosing. OGCS keeps two independent accounts looking alike. GWSMO replaces the account behind Outlook. A personal Google calendar and a corporate Outlook calendar are two accounts, so OGCS is the tool that matches that situation, not GWSMO.

What OGCS actually copies

The project's own site is specific about the feature list, which is useful because generic sync tools rarely say what they leave behind.

Sync can run from Outlook to Google, from Google to Outlook, or in both directions. The attributes it carries include the subject, description, location, attendees with their required or optional status, reminders, free or busy availability, public or private status, and categories with their colours. Recurring items are synced as a series rather than as a pile of separate events. Comparison is differential, so only attributes that changed are updated.

Two settings do most of the practical work. The date range to synchronise is adjustable in both directions, past and future, which keeps a decade of old meetings out of the destination calendar. The sync frequency is configurable, including a push sync triggered from Outlook.

There are privacy controls that suggest the author has used this at work. Chosen words can be obfuscated, and items pushed to the destination calendar can be forced to private, or forced to show as available, so a personal calendar receiving work events does not leak client names.

The requirements are where a Mac user stops. The site lists any version of Windows with .NET Framework 4.5 as the minimum, with the installer adding the framework if needed. Version 2 requires a classic Outlook client, from Outlook 2003 through to Microsoft 365 releases. For the newer Outlook for Windows application, the site states that version 3 must be used, connecting to Microsoft 365 cloud accounts through its Outlook Online functionality. Install rights and a direct internet connection are both listed as not required, and portable builds exist that run without installation.

Why neither tool helps directly on a Mac

Google's download page for GWSMO says it plainly. The executable installer and the administrator package are both described as being for installation on Windows machines only, even though the file can be downloaded from any operating system. OGCS is a .NET Framework application for Windows.

That leaves three honest options for someone on a Mac.

Run Windows somewhere. A virtual machine or a spare PC can host OGCS, and because it holds credentials for both accounts and talks to both services over the network, it does not need to be the machine anyone actually works on. This is the route that keeps full two way sync with attendees and reminders intact. The cost is a Windows installation to maintain and a machine that has to be awake for sync to happen.

Stop syncing and start viewing. Instead of copying events between two accounts, put both accounts into one calendar application and let each event stay where it was created. Nothing is duplicated, because nothing is copied.

Let the servers talk. Availability sharing between organisations happens at the service level rather than on a laptop. It answers the question of whether someone is free, without moving event details anywhere.

What the Windows route really costs

The virtual machine option is more practical than it sounds, and worse than it sounds, in different ways.

It is practical because the sync host does not need to be comfortable to work on. It needs a copy of Windows, network access, and to be powered on when sync should run. Nothing about the experience of using the calendar happens there. The portable builds that require no installation and no administrator rights make this even lighter, since a sync host can be set up without touching system settings.

It is worse than it sounds because of when sync does not happen. A laptop asleep in a bag is a laptop not syncing, and the gap is invisible until a meeting fails to appear. Anyone relying on a local sync tool ends up either leaving a machine awake or accepting that calendars are aligned only after the tool next runs. That is a different reliability story from an account held directly in an app, which is current the moment it is opened.

Putting both accounts in one app

The second option deserves detail, because it is the one that fits a Mac without extra hardware.

Apple's Calendar guide describes adding accounts directly: iCloud, Google, or another CalDAV account, through Calendar and then Add Account, with a manual CalDAV path for providers that are not listed. Google, on its side, documents two ways to connect a Google calendar to another application. Signing in with the Google account gives view and edit. Using the secret address in iCal format for a single calendar gives a view only copy.

Microsoft calendars have a caveat worth reading before committing. Apple's guide notes that for calendars administered through Microsoft Exchange Server 2007 with Service Pack 1 or later, the account has one main calendar, and additional calendars added to that Exchange account do not show updated invitations or attendee responses, are not visible to delegates, and do not appear when other people check availability. Full calendar features are available only in the main Exchange calendar. In other words, a Mac app can hold an Exchange account well, but the main calendar is where the meeting machinery lives.

The refresh settings that decide how fresh it looks

Once both accounts sit in one application, freshness is a setting rather than a mystery. Apple's guide describes two separate controls, and mixing them up is a common source of complaint.

For a whole account, the control is in Calendar settings under Accounts, where each account has a Refresh Calendars menu. Choosing Push means the account updates automatically when a change is made on another device using the same account, or when somebody edits a shared calendar in it.

For a calendar that arrived as a subscription rather than an account, the control is on the calendar itself. Control-click the calendar name, choose Get Info, and set the Auto-refresh menu. A subscription with a long auto-refresh value looks stale in a way that resembles a broken connection but is not one.

There is also a real error indicator, which is worth knowing so it is not confused with the above. Apple's guide states that a warning symbol next to a published, shared or subscribed calendar means Calendar cannot connect to that calendar account, and suggests checking the connection and confirming the calendar has not been deleted or moved.

Arrangement	Fidelity	Runs on a Mac	Maintenance
OGCS on Windows	Attendees, reminders, categories, recurrence	No, needs a Windows machine	A Windows install and a machine left running
Both accounts in one Mac app	Each event stays in its own account	Yes	None after setup
Read only iCal or ICS feed	Times and titles, no editing	Yes	Refresh interval and a private link to protect
Service level availability sharing	Free or busy only	Yes	Administrator work on both sides

The row that suits most individuals is the second one. A desktop calendar that signs in to the Google account and writes to it, while overlaying anything else as a feed, produces one screen with the whole week on it and no copies to reconcile.

One integration is ending in October 2026

Anyone planning around Microsoft's own bridge should know the schedule. Microsoft's documentation for calendar syncing between Google Workspace and Microsoft Teams states that the feature will no longer be supported beginning October 2026, that calendars previously set up for syncing will stop syncing, and that administrators will no longer manage calendar sync from the Admin app within Teams. The same notice points end users who want to schedule Teams meetings from Google Calendar to the Microsoft Teams Meeting add-on for Google Workspace instead.

This is the sort of change that decides an architecture. A plan that depends on a vendor keeping two competing calendar systems in step has a shelf life. A plan where each event lives in exactly one account, and every other view is read only, does not depend on any bridge staying alive.

Decide what has to be true, then pick

Three questions separate the options quickly.

Do attendees and replies need to move, or only times? If a copied event has to keep its guest list and collect responses, only a real sync tool will do, which means OGCS and a Windows host. If the goal is to stop double booking, a read only view of the other calendar is enough.

Who owns the account? A corporate Outlook account under administrator control may forbid third party applications outright, and no amount of local software changes that. Checking the policy first avoids building on something that will be revoked.

How much delay is acceptable? Sync tools run on a timer, and a timer means a window where the two calendars disagree. One account held directly in one app has no such window.

What to change first

Pick one account as the place events get created, and demote everything else to a view. If that writing surface should be a Mac app that holds the Google account with full read and write, shows the rest as feeds, and adds travel time between appointments, Caltimate is built around that arrangement. Then leave OGCS for the case it is genuinely best at, which is a Windows machine that has to keep two full calendars mirrored.