To clarify: The T has announced that they will hold the North Station commuter rail trains until 15 minutes after the game, but they have NOT said anything about special scheduling on the Orange or Green lines. However, FatKidsDad's suggestion is still a good one. With the regular Orange Line shedule, the last southbound train leaves North Station shortly after 12:30. Unless there's OT, that should be plenty of time.
And even if they don't announce it in advance, if the platform is still packed after the last train leaves, it's likely they would add another train or two.