Green’s Dictionary of Slang

a veteran, an ‘old hand’; an experienced soldier, esp. a national serviceman who has completed six or more months of his enlistment or who has completed his entire time.

1970		informant in DSAE (1996).
1979		Sun. Times (Jo’burg) 1 Apr. n.p.: When a national serviceman becomes an ‘ou man’, he’s served on white plates which are washed for him afterwards [DSAE].
1986		J. Whyle in Gray Market Plays 173 : We lived in a large bungalow where we were liable to unofficial initiation by the oumanne. [Ibid.] 175: I had nearly done a year in the army and was due for a seven-day pass. The roofie ouman tension seemed to have abated and we were almost on friendly terms [DSAE].