The FAQ on the official Japanese website for MGSV:TPP has been updated with some new information on online play, which has been translated by JunkerHQ.
1. In MGO matchmaking, you can choose between ‘Japan’ or ‘ROW’.
2. You can infiltrate other people’s bases and they can infiltrate yours even if you don’t have PlayStation Plus or Xbox Live Gold.
However, you won’t be able to deploy to defend your base without the online services (and you’ll need it to play MGO as well).
3. Mother Base resources are kept separate from those from your FOB.
4. You won’t be able to use vehicles when infiltrating FOBs, and you can’t bring a buddy along.
Source: Official Japanese MGSV site, translation by JunkerHQ, via MGOF