Nextcloud forked from the old PHP-based ownCloud stack, while Opencloud forked from the Infinite Scale Go-based stack. It also by default preserves the filesystem hierarchy on your server without needing a database, using a storage driver called PosixFS.

The Windows clients currently do support selective syncing so it is on-par with OneDrive. Android client looks to be forked from old Owncloud, and has offline availability too.