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OneCalendar vs Fossify Calendar: catalog facts
OneCalendar	Fossify Calendar
Free Free to download from the app stores.	Free Free and open source with no ads.
Windows, macOS, Android, iOS	Android
Supports Google, Outlook, Exchange, iCloud, CalDAV and many other providers Available on Android, iPhone, iPad, Mac and Windows	No ads or tracking Syncs with Google, Outlook, Nextcloud and Exchange calendars
Closed source No Linux version	Android only Sync relies on accounts already set up on the device
Downloadable app	Downloadable app
Closed source	Open source
License not stated	License: GPL-3.0-only
Not stated if an account is needed	No account needed
Works offline	Works offline
Official website Full listing and sources Checked October 2, 2026	Official website Full listing and sources Checked October 2, 2026

Catalog facts only. Anything “not stated” is unconfirmed. Check full listings for details.