What is the model and year of manufacture of a 15 HP Mercury with the serial OG123138?

A 15 hp Mercury outboard, serial number OG123138, would be a 1995 year model. This is all the information that can be retrieved from the info given. As far as a particular model, in a small outboard, it could be either a 15M, (manual start), 15E, (electric start), 15ML, or 15EL, the "L" denoting a longshaft model.