The 30 year old is still looking for a maiden European Tour title but gave himself every chance as he set a 15 under par target at East London Golf Club.
Challenge Tour graduate Robert Dinwiddie and his fellow Englishman Chris Wood were best placed to deny O'Hara - both sitting 15 under with two and four holes to play respectively.
Seven different players had already been top of the leaderboard on an enthralling final day - and Spaniard Manuel Quiros and Open Champion Louis Oosthuizen were both 14 under with six holes remaining.
Miles Tunnicliff, Jaco Van Zyl, Markus Brier and Branden Grace were also in the running as they sat 13 under with holes to play in what promises to be an exciting finish.
|T4||VAN ZYL, Jaco||RSA||18||-15|