Three goalkeeping errors by Dean Fuller cost goals by Zak Newman (23 & 30min) and newly-signed Crawley Town youth player George Smith (84) for their second-division oponents. But United boss Ian Browne abandoned an unproductive 4-3-3 at the interval.
Switching to 4-4-2 and swiftly introducing the pacy Dave Schneider turned the game on its head. Sonny Banks (51), Schneider (56) and Stuart Sell (63) with a deflected shot, made it 3-2 to United. Then Shelley beat four players and slipped in Schneider for a dramatic last-minute winner.
Browne said: "Changing system instilled some hunger, passion and desire throughout the team. Oakwood attack you quickly but give you a chance at the back and I thought goals would come. Schneider's impetus and work rate made us more dangerous."
Oakwood manager Bob Rye admitted: "We were naive in the second half and United upped their pace and came at us. We couldn't cope with them and they hit us where it really hurt."
UNITED: Fuller; Hughes, Johnson, S.Di Paola, Keech; Bone, Webb, N.Shelley; Brown, Banks, Sell. Subs: Schneider (Bone 47), Footham (Webb 76), Warren, Butler gk. Att: 63. Cameron Johnson's debut. Previously with Worthing, Littlehampton, Lewes and Burgess Hill.