I was wondering if someone could explain the difference in these titles. My understanding is that the "CO" :- is the commanding officer who is appointed to command a unit. ( Be that by Congress or Royal Warrent ( Commision). However sometimes an Officer finds himself as the OC as they are the senior officer. Thus an OC of a regiment could be a Lieutenant, because his CO could be absent (or dead). I'm thinking WW1 here. How does it work in the modern era.

Ok – Comedy relief – having a psychologist for a wife the OC crabbed my attention. OC stands for Obsessive Compulsive. Gotta have things just so! I can see being in this hobby with OC – gotta have just the right # of rivets or just the right thickness armor.
CO = Commanding Officer, who in themselves probably has a bit of OC in them.

To Funny Scott!!!!!!!!!!!
To answer your question Scharfuhrer.
CO = Commanding Officer.
OIC = Officer in Charge
Usually, and officer is apointed in charge of a task. For example, when i was in germany, i was the unit mail clerk. An officer was apointed OIC of the mail room, meaning he or she reported to the Commander all inforamtion and anything related to the mail room. Antoher example would be during a convoy. If the Commander is not present, he or she would apoint a person in charge of that convoy, Officer in Charge(OIC). There would also be a NCOIC (Non Commosioned Officer In Charge).
I hope this explained a little bit, and I'm sure the other military guys will add the little info to or even correct me if I am wrong.

Epi,
Good stuff - Thanks. Clear as a bell.