oCPM (optimised Cost Per Mille) bidding at Ad Set level on Facebook allows Facebook to optimize the impressions it shows to drive more of the conversions you specify for a fixed budget. This is generally one of the most efficient ways to bid on Facebook, because it allows Facebook to use all of the data points available to the Ads Platform to spend as much money as possible - and Facebook knows you will only continue to spend that money if the results are good. In order for Ad Sets to improve performance Facebook generally recommend that targetable audience size should be 1 million and the Ad Set should be driving #25 conversions per day. If the audience size is too small it should be increased to allow Facebook to optimize effectively.