Getting his service records should give you alot of information. My ancestor was in the 14th (in the same Brigade as Orr's) and his records are pretty robust compared to some ancestors in another regiment.

SC has Confederate pension records on-line

http://www.archivesindex.sc.gov/Archives/default.asp

FYI, since he was a South Carolinian, I would suggest you run his name through all the on-line records. You may be amazed at what you will find.

I would also recommend that you procure a copy of The History of a Brigade of South Carolinians, by Caldwell, which written during the war about McGowan's Brigade, which was Orrs Rifles brigade. Caldwell's book is no longer under copyright and there are reprintings available. I got mine from:

http://www.researchonline.net/catalog/books4.htm

The Brigade was camped at what is now Pamplin Park in Petersburg, VA. They have existing earthworks built by the Brigade and have "decorated" the house where the McGowan and staff stayed to look like it probably did during the time period.