Optimized CPM (or oCPM) was (or still is?) a type of bidding strategy Facebook made available circa 2014.
It is an optimized version of the CPM model: once your ad enters in the auction, Facebook will show it only to those users most likely to take the action you want (clicks, plays, etc.)
It is not surprising that it will come with an additional cost: you are paying a premium because you are now in a special bucket, those advertisers willing to spend more per impression.
If you don’t have any CPM value baseline, your best strategy is to set a cost cap: how much are you willing to pay for results?
I doubt you would want to pay $100 per click (maybe you do), but defining this limit will help you to get the results you want for the budget you have.
Merry Christmas! 🎄