In dentistry, what is an MO and an ML?

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.
Surfaces on teeth. An MO is mesial and occlusal, and ML is mesial and ligual.
Tooth surfaces. MO = Mesio-occlusal and ML = Mesio-lingual.