Looks like Capt. John O'Brien may have officially belonged to Co. F, 25th AR Infantry. (Due to combining units, the 25th AR Infantry was temporarily designated as the 30th AR Infantry during '62 and at Murfreesboro, but reverted to its original name again soon after this engagement).
Reading between the lines here. Am guessing that O'Brien was captured at, or shortly after, Murfreesboro. This might explain why he more readily identified with the name of the 30th AR (not 25th AR) as his unit, for the purpose of his writings.