Hm, depends what part of Office 365 you need, the offering is truly gigantic. If you need something like Word Online, there's some stuff floating around (search

Hm, depends what part of Office 365 you need, the offering is truly gigantic. If you need something like Word Online, there's some stuff floating around (search "open source google docs" to find them). As for the rest, Exchange Online, Sharepoint, especially how it's all integrated, I'm afraid there's simply no alternative. Nextcloud offers some integration in that regard.