Metal Gear Solid V: The Phantom Pain's FOB mode is a microtransaction.
Looks like you can unlock it by beating single player missions or paying real money.
Konami has moved to clarify how microtransactions in Metal Gear Solid 5 work after reports indicated the FOB online multiplayer mode was locked behind a paywall...yesterday Gamespot reported players would have to spend real world money to build a Forward Operating Base (FOB), which can be invaded and defended by those who choose to enable the mode...mention of a "paid service" set tongues wagging and sparked concern among prospective Metal Gear Solid 5 players
Konami later clarified further with Gamespot, insisting the "initial" FOB platform is not behind a paywall. Here's the follow-up statement:
To clarify, the initial FOB, which includes the FOB command platform and all sub-platforms, is not behind a Mother Base coin paywall. Maintaining other FOBs past the initial set of FOB platforms can be purchased with Mother Base coins. More information regarding the payment model for FOB will be provided closer to launch