oCPM, or Optimized Cost Per Mille - Allows for bidding based on your desired end goal (objective type). It's used to drive website conversions, specific engagement actions, or app installs. Based on your oCPM bid settings, Facebook will use their algorithm to adjust the ad placement, frequency, and reach that best complements your objective type. This often results in the lowest cost per action (the action being your objective type) compared to CPC & CPM bid types, in many cases.