Orifice Openness is one of the bimbo stats of the player. It measures how well used your butthole and vagina (if you have one) are. They slowly return from loose to tight as time progresses. Whilst these stats do add to your bimbo score, they also reduce the chance of soreness increasing when you are being penetrated, so one could argue that these stats are worth keeping high! The score of each stat goes from 1 to 10.
Every turn that the player is being fucked or otherwise occupied by something larger than the current openness level, there is a chance for openness to increase. The chance increases the larger the insertion is..
The orifice openness of the player decreases by 1 every 400 seconds. If the player is female, the orifice with the larger value is the one that decays.
When the game attempts to increase the soreness of an orifice, it compares the openness of the orifice to the size of the insertion to calculate the chance that soreness increases this turn. So the more gaped your orifice is, the slower soreness will increase.