Cheyenne Dunkley, Chris Maguire and and Callum O'Dowda scored second-half goals at Kassam Stadium to give the home side a thumping win.
The resulting three points saw them leapfrog Accrington Stanley - who were held 0-0 at home by Stevenage - and go into second place, clinching automatic promotion.
And agonisingly for Stanley fans, Accrington were also overtaken by Bristol Rovers, who won 2-1 against Dagenham & Redbridge.
AFC Wimbledon beat Plymouth to secure League One promotion
Rovers looked like they would come up short, as their match entered injury time with the scores locked at 1-1.
But Lee Brown scored to give Rovers the third automatic spot in League One, while Accrington go into the play-offs with Plymouth Argyle, Portsmouth and AFC Wimbledon.
"It means the world to me. It means so much to so many people," said Brown.
"It just happened to fall to me. I was the lucky one. I'm just so happy it hit the back of the net.
"What a group of players, I can't rate them highly enough. It's going to be a long night."
Dagenham & Redbridge go down into the Conference, alongside rock-bottom York City.
Penalty levels League Two play-off first leg between Portsmouth and Plymouth Argyle
Hughes winner keeps Accrington on course for automatic promotion