After the resulting scrum the prop Andy Coley, who makes his Lancashire debut in tomorrow's Origin match, bundled aside two tacklers and put Malcolm Alker over.
Things became critical for the Wallabies at the scrum with prop Ben Alexander sin binned in the 24 minute and the penalty was kicked once against by Halfpenny to establish a 19-3 advantage just as Israel Folau appeared to hurt his hamstring reaching for a long kick ahead by the Lions.
Muldoon profited from a scrum drive, prop Ah You also went over from close range while veteran Dragons lock Robert Sidoli was in the sin-bin for illegally taking out a Connacht opponent at a line-out.
But despite the Ospreys having the edge at the scrums, home prop Thomas was able to celebrate a well-taken try.
From the resultant scrum, the 135kg prop won a penalty which Horne slotted over to break the deadlock.
Worcester feel the cost of all those scrum penalties as prop Nick Schonert is shown a yellow card.
The working of these laws means the roles in the scrum of hooker, prop and so on are distinct, and require different physical attributes.
Well after that warning from the ref, it appears like David Humpreys has seen enough to realise Paddy McAllister can't deal with Kieran Brookes at the scrum and the prop comes off for Yann Thomas.
In the scrum, England is without prop Andrew Sheridan, while prop Nicolas Mas returns for France.
If scrum-half Frank Murphy, prop Marcos Ayerza and Alesana Tuilagi had cause to wince inwardly for not paying more attention, it was nothing to the horror of the coaching staff when virtually the same thing happened again in the second quarter, Simon Shaw's deft return pass putting an ecstatic Ibanez over in the corner.
But Saints - without England captain Dylan Hartley, Lions wing George North, scrum-half Lee Dickson, prop Kieran Brookes, flanker Tom Wood and number eight Sam Dickinson - pushed the favourites before running out of steam in the final 20 minutes.
