Damien Varley opened the scoring after 4 minutes with the pack driving over the line, O'Gara adding the conversion, but Northampton were quick to respond with a Chris Ashton try 4 minutes later. Doug Howlett added Munster's second try in the last play of the first half.
Northampton went ahead by a point, 20-21, after a James Downey try on the 56th minute and the visitors held on to that lead until the final kick of the game coming from the boot of O'Gara.
Another historic milestone was reached in Thomond Park this evening when John Hayes came on in the 70th minute to mark his 100th appearance in the Heineken Cup.
Munster: J Murphy; D Howlett, D Barnes, L Mafi, D Hurley; R O'Gara, C Murray; W du Preez, D Varley, BJ Botha; D Ryan, P O'Connell (capt); P O'Mahony, N Ronan, J Coughlan. Replacements: D Fogarty, J Ryan, J Hayes, D O'Callaghan, D Leamy, T O'Leary, I Keatley, W Chambers.
Northampton Saints: B Foden; C Ashton, J Clarke, J Downey, V Artemyev; R Lamb, L Dickson; S Tonga'uiha, D Hartley (capt), B Mujati; C Lawes, M Sorenson; C Clark, T Wood, R Wilson. Replacements: M Haywood, A Waller, P Doran-Jones, S Manoa, P Dowson, M Roberts, S Myler, G Pisi.
Referee: N Owens (Wales)