Deferrals: MEL (Minimum Equipment List) and POH For Cessna 172N '78

Hello-

I am a student pilot getting studying for my checkride. One question
that I want to get right is the issue mentioned in the subject header
above.

My instructor said that because we do not have a MEL, we use the POH
and anything that is considered Optional but is defective can be
deferred and the flight could be made. There are three ohter
designations in the Equipment list, "A" for additional, "S" for
substitiuded and "R" for required. I would think that the "A" stuff
probably could be deffered as well (ie AC equipment, autopilot, etc).

I am a bit unclear and wanted some clarification on this from the
group. Does this mean to refer to the equip list in the POH and
anything with the letter "O" or optional that is defective can be
deferred and the flight can be continued? Or can the stuff "A" and "O"
be deferred.

Thanks
SD.