The OCC renders OFMs unnecessary-I'm unaware of whether the OFM was intended to have a new function, or was simply left in by mistake. I'd lean towards the former, if only because I find it unlikely that it was both left in by mistake and given a new function by mistake.
If I remember correctly (can't check at the moment, sorry), there are a number of fields that "cut off" before showing the actual value that they give, and planetary defense is one of them.
Double checked the XML files (because I hadn't actually built a planetary defense) and it is indeed missing the tag that grants it a bonus to planetary defenses, so it's not just a display bug.
Also, for the record, it's not a soldiering bonus. Essentially, it counts your troops as X% more for combat purposes-so one at 25% on an 8B planet would calculate your combat losses as though you had 10B pop on that planet.
Orbital Command Center is unlocked from Interstellar Construction, which is generally right before Advanced Hulls.