Trailing 21-20, Munster went back and forth through a mammoth 41 phases as they tried to get the tournament's record points scorer into drop-goal range.
Saints kept their discipline and defended manfully but O'Gara showed his mettle with a superb long-range strike that cruelly denied the visitors a famous victory on Limerick soil.
Doug Howlett and Damien Varley scores tries for Munster with James Downey and Chris Ashton the men to cross for Northampton.
Defending champions Leinster salvaged a dramatic 16-16 draw in a thrilling Pool Three clash against fired-up newcomers Montpellier.
Montpellier captain Fulgence Ouedraogo finished off a breakaway try midway through the first period at the Stade de la Mosson as the hosts led 13-6 at the break.
Francois Trinh-Duc extended Montpellier's lead early in the second half but in the final quarter Leinster finally began to wear down their opponents.
Second half replacement Sean Cronin barrelled over in the right corner with 14 minutes remaining. Despite Sexton's successful conversion, it looked like Leinster would have to make do with a losing bonus point.
But right at the death referee Dave Pearson awarded the visitors a penalty and Sexton held his nerve amid a cacophony of boos to grab a share of the spoils.
Dan Biggar delivered an immaculate goal-kicking display as the Ospreys launched their Pool Five campaign with a 28-21 victory over French heavyweights Biarritz.
Former England wing Iain Balshaw scored two tries for Biarritz, but his opposite number Tommy Bowe's second-half touchdown - and 23 points from Biggar - ensured a winning start in Pool Five for the Welsh regional side.
Leicester kicked off their campaign with a well-earned 28-12 victory over Aironi but missed out on a potentially crucial bonus point.
With Clermont Auvergne and Ulster making up Pool Four, the two-time European champions would have targeted a maximum-points haul in northern Italy.
But despite dominating the first half they failed to grab the necessary four tries.
Tom Croft and Alesana Tuilagi crossed for tries in the rampant first half but Leicester took their foot off the gas following the interval and it was not until the 80th minute that Niall Morris scored their third try.
Ian Humphreys scored all of Ulster's points as the Irish outfit began their Pool Four campaign with a hard fought 16-11 win over Clermont Auvergne.
The fly-half was successful with four kicks and scored a crucial second-half try as the hosts turned the tables after trailing for the vast majority of the contest at Ravenhill.
A Clermont side packed with international quality could only muster an early try through Seru Noa Nakaitaci and two penalties via the boot of David Skrela, but did secure a losing bonus point.
Edinburgh came from behind to earn a deserved 20-19 Pool Two victory over London Irish at the Madejski Stadium.
Irish led 19-10 after 56 minutes, but fell away poorly in the second half to lose the Pool Two encounter.
Lee Jones and Stuart McInally scored the visitors' tries, while Ross Samson crossed for the home side.
The Scarlets got their Pool One campaign off to an impressive start with a hard-fought 31-23 victory over Castres at Parc y Scarlets.
Second-half tries from Sean Lamont, Ben Morgan and Jonathan Davies saw the Scarlets celebrate their 100th European game with a win, as Stephen Jones also added 13 points with the boot.
Castres' tries came from Marc Andreu and skipper Chris Masoe, with Romain Teulet kicking 13 points.
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.