After half-time, Mafi was sin-binned after a high tackle on Rob Kearney but O'Gara kicked the 14 men back in front after Gordon D'Arcy had transgressed at a ruck.
Johnson's side clawed themselves back to within a point but again played 20 minutes with 14 players after the sin-binning of Danny Care and Phil Vickery.
On 59 minutes Bristol lost Crompton to the sin bin after the prop tackled Tom Shanklin high, although Blair was short with the resulting long-range penalty attempt.
Gloucester carried that momentum into the second half and after Ben White was sin-binned for slowing down the ball, they moved the ball right after a quick-tap penalty for Simpson-Daniel to score and Robinson converted from the touchline.
Nieto had long since returned after his first-half sin-binning but his discipline let him down once more 10 minutes after half-time as he again infringed at a ruck and O'Gara made it 11-0.
Bradford, who had Sam Burgess sin-binned after 32 minutes, finally came to life after half-time.
But he followed it up by being sent to the sin-bin after back-chatting the referee once too often.
Tempers started to flare on the field and Jamie Peacock and Josh Perry were both sin-binned after 20 minutes.
After McAlister was sin-binned early in the second half, Thierry Dusautoir took advantage to score and level the game.
Ulster blindside flanker Thomas Anderson was sin-binned after he was adjudged to have gone over the top of a ruck.
Indeed, Irish prop Marcus Horan had been penalised on a couple of occasions and was sin-binned after another indiscretion late on.
Webster went close again but was thwarted yards from the line before Blues scrum-half Richie Rees was sin-binned after deliberately knocking on.
Saracens had replacement Andy Saull sin-binned after 69 minutes, and Gloucester made sure they kept their noses in front for a valuable win.
When Tonga saw former Newcastle and Sale player Taione sin-binned after yet another infringement at a ruck, a fourth Williams penalty made it 12-3 after 28 minutes.
But it was Tomkins' brother Joel who got into worse trouble right on the half-time hooter when he was sin-binned after a push and shove on the halfway line.
The 30-year-old added that he was not happy with his sin-binning after coming on as a replacement in the second half of the 16-13 Magners League defeat by Edinburgh at Ravenhill.
Just before the hour mark Italy lost prop Martin Castrogiovanni to the sin-bin after he infringed with England five yards out and Wilkinson edged them into a 14-6 lead with a simple kick.
The visitors' points came from fly-half Colin Gregor's penalty brace in a game in which Blues prop Scott Andrews was sin-binned after 29 minutes and Glasgow's Paul Burke was yellow-carded 11 minutes from the end.
Burgess found himself in the sin-bin after getting involved in a scrap initially sparked by Matt Petersen squaring up to Newton in the 32nd minute, but when he came back on shortly after the restart Bradford finally clicked.
The Tigers' hopes suffered a major blow when replacement Michael Korkidas was sin-binned just after coming on.
And the Scotland stand-off reduced the deficit further to a single point, again from a penalty, after Allan Jacobsen was sin-binned for an over-aggressive clear-out.
The Scots then lost Paterson to the sin-bin, after the full-back held onto Martyn Williams in the tackle as the Blues threatened to break through again.
With the Blues seeking a bonus point the visitors' resolve seemed to strengthen, though, and after seeing out the sin-bin period flanker Burban struck back, shrugging off Thomas in a bullocking run down the flank for his side's second try.
The Ospreys continued where they left off the previous weekend at the Liberty Stadium, with Parker, Wyn Jones and Thomas crossing in the first 25 minutes in which the hosts were down to 14 men for 10 minutes after Hottie Louw was sin-binned for illegally taking out James Hook.
The Americans were then reduced to 13 men after flanker Todd Clever was sin-binned for persistent infringements at the breakdown.
BBC: SPORT | Rugby Union | Internationals | Ireland A start with win over USA
The Ospreys' wobble continued after the break, Phillips sin-binned for a trip on opposite-number Blair as the scrum-half took a quick penalty.
French international Parra could not convert and Hougaard again missed a chance to close the gap after Davit Zirakashvili had been sin-binned.
Byrne was in danger of going from hero to zero after his second-half sin-bin but the 14-man Ospreys defence kept the Blues at bay.
Both teams were down to 12 men in the final 10 minutes when Jamie Peacock and Adam Peek were sin-binned for fighting after a scrum.
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