Ord MAM stands for Ordnance Medium Automotive Maintenance, and is usually a description of a company or batallion (Bn). In the regular army, they had units that were in support of divisions to fix artillary and vehicles, etc.
Being part of an Ordnance unit or company could mean they had a wide range of jobs. If you are looking for what they specifically did, if you can get ahold of the separation (discharge) papers, they usually list his Military Occupation Specialty (MOS).
I think being part of the Army Air Force, an Ordnance person probably handled bombs and such. I don't know if they were part of a particular 'crew' or if they were attached ground support units to the bomber groups, squadrons or wings.
I relatively new at all of this as well, I hope this has helped.