The ogre is fighting vs 7 submerged PCs and it's a trap. Here is what usually follows;
Wizard does an offensive block on the Ogre, blocking Perception (blinding it).
2 mortals firing with skill 5 weapon 3 shotguns, having aimed and spending 2-3 fate points each to hit plus +1 from the Ogre's hulking size with cold iron shot. Ogre is attacked with a to-hit roll of +14, weapon 3 and its dodge is 5. It takes 2x 12 stress which ignores its toughness.
This is at least 6 fate points to just survive the above attack. The party attacks from 2 zones away, which means the ogre must take at least 1 whole exchange to get into melee - and the party gets to do another full attack at it, possible with all members.
I don't mean to derail, you did ask a specific question (and yes, the answer "X-1 where X is the number he'll need to just beat them" is pretty good) but is it possible that he could call in favors to get an assist from the Nevernever? Even with three PCs I've had a lot of trouble with one BBEG taking on the party, and I imagine that seven fully prepped PCs will make short work of him.
(and more importantly who pulled his strings to get him to do it)
There you go. I mean, if there is some sort of higher level to the antagonists,that higher up could send his/her own critters to save the ogre and/or kill the ogre before he spills his guts. That actually works out even better, since if the PCs did a really good job setting this guy up they should take him down easily. This way you encourage that sort of play but also get a big climactic confrontation, and can give the PCs a lead on the Ogre's boss, assuming the type of minions he/she/it sends are a hint.(click to show/hide)