Grand Slam star Williams is better known for his tries but produced a remarkable left-footed drop goal from just inside the half-way line to cap a resounding 31-10 win for the Ospreys in the Magners League.
The Ospreys had to come from behind to take the four points against a typically gutsy Connacht side that led at half time. Mark McHugh was off target early on and it took half an hour for either side to manufacture a point - when Warwick exchanged penalties with Shaun Connor before finding the key to unlock the Ospreys' defence on thirty-eight minutes.
The fly half jinked inside All Black Justin Marshall and shrugged off the attentions of seventeen-stone Osprey captain Paul James before slipping his pass for Slemen to sail over. Warwick converted for a seven-point advantage that lasted barely two minutes.
The Ospreys were galvanised into action by the visitors' score and Lee Byrne rolled out of McHugh's try-line tackle to draw his side to within a score. Connor missed the chance to equalise with the conversion and Byrne took over the kicking duties after the break, slotting over two penalties.
Connacht were still in the game at 14-10 but replacement Matt Lacey left his team a man down after receiving a yellow card as the Ospreys turned the screw - Williams was twice involved in the attack before Lacey was penalised for not rolling away.
The Irish looked to have snuffed out the immediate danger when Colm Rigney stole posession yards from his own line but the Ospreys tore it back and James crashed over between the posts.
The Connacht scrum buckled five minutes from time and Scottish referee Andy MacPherson awarded a penalty try to cap a successful night all round for the Ospreys.
Ospreys 31 Connacht 10
Tries: L Byrne, P James, Penalty
Cons: L Byrne (2)
Pens: L Byrne (2), S Connor
DG: S Williams
Try: D Slemen
Con: P Warwick
Pen: P Warwick
Ospreys: L Byrne (C Martenko, 78); N Walker, T Selley, A Bishop, S Williams; S Connor (J Spice H-T), J Marshall; P James (capt), H Bennett (B Williams, 75), A Millward (A Jones, 51), B Cockbain, M Powell, L Beach (T Smith, 72), J Thomas, R Pugh
Connacht: D Slemen; D Yapp, J Hearty, M McHugh (P Durcan, 74), D Riordan; P Warwick (G Williams, 67), C O'Loughlin (C Keane, 67); R Hogan (B Sturgess, 74), J Fogarty (capt, A Flavin, 67), S Knoop, D Gannon, R Strudwick (M Swift, 71) , J Muldoon, R Ofisa (M Lacey, 47), C Rigney
Referee: Andy MacPherson (SRU)