Hannah Short pulled one back for hosts Oxford but a brace for Christie Murray and a further effort from Jess Sigsworth earned the Belles their first points in the cup this season.
Belles can’t reach the knockout stage of the competition but the victory means they go into Wednesday night’s final group game at home to Manchester City in good spirits.
Rayner’s first – a header from Murray’s corner – required a bit of good fortune, with the linesman indicating the ball crossed the line as goalkeeper Claire Skinner caught it.
The striker’s second owed nothing to chance as she found the bottom corner after Murray’s good work down the right created space for Samantha Tierney to run into before finding Rayner.
Oxford pulled one back in the 12th minute through defender Short, who rose superbly in the box to meet Dan Carlton’s free kick and nod past Nicky Davies.
Murray went close with two lobs from the edge of the box – one either side of the interval – but saw both drop over Skinner’s crossbar.
But the Scotland international did not falter from closer range, however, slotting past Skinner from substitute Monique Watson’s pass to kill off the game in the 69th minute.
Winger Sigsworth put the result beyond all doubt with a wonderful top corner finish with the outside of her right foot from Maz Pacheco’s cross.
Murray added the gloss with her second goal four minutes from time with an excellent 25-yard strike.
Belles interim head coach Kate Rowson said: “It’s good that we had that good start but the 20 minutes that followed ended up being a little bit frantic.
“I thought the whole game was a little bit too stretched for my liking but we can’t complain. We were up from the very start and up at half time, and then finished it off in the second half.
“Leading into the game we discussed how, when we get the lead, we need to manage it.
“It just seemed a little bit frantic more than anything. We calmed it down and in the second half they really applied what was said at half time, and that was the biggest joy out of today.
“They recognised ‘Oh yeah, this isn’t quite right, let’s apply it’, and fair play to them because they did.”
Belles host Sheffield FC Ladies on Sunday in FAWSL2 (2pm).
To buy tickets to an FA Women’s Super League match near you visit www.fawsl.com/tickets.