In dentistry, what is an mo and an ml?

Tooth surfaces: MO = Mesio-occlusal and ML = Mesio-lingual

Surfaces on teeth: An MO is mesial and occlusal, and ML is mesial and ligual.

Location location: The letters you are asking about are tooth locations. They tell us where on a tooth a problem is located. MO is Mesial/Occlusal. ML is Mesial/Lingual. B is Buccal. D is Distal etc. Dentistry does have it's own lingo.

Surface of filling: MO and ML is an abbreviation of the surface or side of the tooth that a filling is placed. The tooth usually has 5 surfaces that are designated during the placement of a filling. MO stands for mesial occlusal. ML stands for mesial lingual.

These are the surfaces of posterior teeth. They are mesial, distal, occlusal, buccal, and lingual and all five could be involved in one filling.