There was confusion before kick-off when it emerged Leeds and England hooker Steve Thompson was missing from the squad despite having been named in the starting XV.
But any trace of doubt amongst the visitors soon vanished as Leeds came flying out of the blocks.
After Oakley turned the ball over in midfield, Mike McDonald went close before Fourie crashed over inside the first five minutes as Adrian Jarvis added the extras.
With eight minutes gone Oakley touched down under the posts and Jarvis made it 14-0.
Saints came back at Leeds and won a penalty that Shane Geraghty converted inside the opening quarter of an hour but Jarvis soon restored his team's 14-point advantage with his boot.
Phil Nilsen looked to have missed a golden opportunity when he tried to offload to MacDonald with the line beckoning but it was the superb Oakley who again rapidly turned the ball over to create the opportunity for Wackett to score.
Jarvis landed the conversion to make it 24-3 with 20 minutes gone in front of a stunned Franklin's Gardens.
Leeds and England flanker Hendre Fourie (with ball) was outstanding throughout at Franklin's Gardens
Wingers Lee Blackett and Paul Diggin were sin-binned for a prolonged dust-up on the half hour as Northampton tried to capitalise of a tiring Leeds defence.
The pressure told when Saints skipper Dowson crossed in Blackett's corner after Saints had stretched Leeds but Geraghty sent his touchline conversion attempt wide.
Leeds went down to 13 men when prop Juan Gomez was yellow carded after a series of collapsed scrums on the Carnegie line.
Saints opted for another scrum and when the front rows came up referee Andrew Smalls awarded the penalty try.
The hosts brought scrum-half Lee Dickson and flanker Calum Clark on for the second half and immediately went on the attack against 14-man Leeds - with Gomez still in the bin.
Carnegie could not hold onto possession and their defence looked strained as Saints began to utilise their powerful bench.
The hosts visibly upped their game with Dickson's inclusion an obvious improvement and a try looked inevitable when Christian Day spilled a pass in a promising move.
Scott Mathie cleared the Leeds lines but his players were too tired to chase and Saints ran the ball back and put Ashton away.
Geraghty converted, Gomez returned for Leeds but the signs were ominous for the visitors with the score at 24-22 and Leeds needing at least two points to stay up.
Myler replaced Geraghty soon landed a peanlty to give the hosts the lead for the first time with 13 minutes of the second half gone.
Leeds needed to stay within seven points of Saints and score a fourth try but it was all one-way traffic with Northampton totally dominant over an exhausted-looking Carnegie side.
As the final quarter of an hour approached Leeds looked out on their feet but strived to make inroads with the scraps of possession they had.
Myler landed a fine penalty with nine minutes remaining and a minute later Saints were queueing up to score as Ben Foden broke the line only for the England full-back to send out a wayward pass.
Myler kicked another penalty with five minutes left but with Leeds still needing a try their last-gasp scramble on Northampton's line proved a lost cause for the Yorkshiremen.
Northampton Saints director of rugby Jim Mallinder told BBC Radio Northampton: "We were terrible in the first 20 minutes, our defence was a shambles, we couldn't look after the ball and it was probably one of the worst 20 minutes we've played.
"We slowly got on top and in the second half, we dominated the game.
"I feel very sorry for Leeds, they've battled throughout the season but they've got to work hard in the first division and come back stronger.
"Leicester is something to look forward to. They'll be favourites, without a doubt."
Leeds head coach Neil Back said: "It is tough to take. We had an incredibly difficult task coming here against the Heineken Cup finalists, with the form and quality they have got in their team.
"They had more than £1m-worth of players on their bench, and ultimately it made the difference.
"But I am proud of our whole squad, as I have been all year. Leeds have never failed through lack of effort, ultimately it was because we were not good enough collectively.
"We played well, we executed simple plays very well and put them under pressure. But we haven't converted enough games into wins, and what has happened has happened.
"We were 11 points adrift at Christmas, and to still be in it now is a mark of strong mentality, character and a willingness to give everything, and that is what the team did today."
On the Championship play-off final: "I didn't watch Worcester's semi-final game against Bedford [which Worcester won by a point] but I think that has been their scare.
"I think they have too much quality in their team that they won't let it happen again.
"Over two games I can only see one winner. My money, if I was a betting man, would be with Worcester."
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