Steve, An overseer was a supervisor (watch over others work) of a farm, Ranch or Plantation in this time frame. I also found a George W. Thomason in Co C of the 18th Texas Inf. (Ochiltree's). He died of disease as a prisioner of war on 23 Aug. 1863. I can't find anything on George A. at Camp Nelson. George W. may have been captured at the Battle of Arkansas Post, Jan. 10-11, 1863.