If a MSFT patent has any relevant to open-source programs, such as VFAT, they're now open to other OIN members.
Even if you got a license for a patent for yourself and wrote free software for it, if someone else gets it they might need a patent license too, regardless of your project being free software.
Your FOSS/FLOSS license might include a patent clause but that means you'd first need the right to issue licenses for the patent, in case you don't, the clause is void.
When they donated the IP to OIN, for all intensive purposes the patents were liberated given OIN's wide coverage.