For your trainees I'd stick to a list of codes that are machine specific that are commonly used in the programs you run.
What I've done to train operators that have basic skills is to take a program that runs on one of our Fanuc machines and add comments to all the lines. I took the same program and converted it to run on the OSP controls and commented it. I included at the begining a very basic glossary of all the terms used in the comments; Reference Planes, Work Offset Coordinates, G81 and it's addresses defined, G84 and it's addresses etc, very basic but every commented term can be looked up in a glossary. With the working definitions and the commented programs they can define each line they are reading. I stayed basic with it, four pages of programming for two controls and two pages of glossary, but it allows them to read ahead and proof a setup with established programs. If they want to go further than that then tech school or as pdoherty said Peter Smid writes some pretty good books.
As far as what psychomil said take a look at the example below.
OSP P200M(ill) control
G98 = Spindle Oil Mist for near dry machining
G71 = Set Reference plane return when M53 is read in a canned cycle. Similar to Fanuc G98
G54 - G59 =Tool comp x;y;z;4th axis;5th;6th so instead of a G43 you use a G56.
G15Hnnn (001-999) = Select Work coordinates system, 1000 standard (eat your heart out Fanuc!)
Suppose you were an idiot and suppose you were a member of Congress. But I repeat myself.
Mark Twain