The U’s took the lead in the first half through Ruben Rodrigues, but a Sinclair Armstrong strike just before the hour mark levelled matters.
The defining moment in the match came with less than quarter of an hour to go, when Mark Harris was adjudged to have fouled Haydon Roberts inside the penalty area.
Nahki Wells stepped up from the spot and smashed the penalty home to complete the turnaround for the Robins.
United head coach Buckingham felt his team could’ve done more to try and kill the game after taking the lead.
He said: “It’s us adapting to the level. It shows the key moments in the Championship. I think we should come away from here with something.
“We had a big chance when we were 1-0 up, to make it two, and I’ve just seen a replay of what is supposed to be the penalty that won them the game.
“There’s a big touch from Mark Harris before there’s any other contact as far as I can see, so it’s very disappointing but it is what it is.
“We can’t do too much about it now, we just need to dust ourselves down and get ready for another home game at the Kassam.”
Buckingham added: “We went one up, and then Tyler [Goodrham] had another at the back post if he gets on target, it goes in.
“We got the goal, and for the chances they had, there’s nothing clear-cut in the chances they had other than the goal that they scored, which is the cutback.
“Everything else, we managed. Jamie Cumming hasn’t really had to make a real save, but that shows the difference at this level.
“In the first half, I think we were the better team. In the second half, they made a couple of changes, got their goal, and we just need to be a little bit better.
“I don’t think we played poorly or that we’ve done poorly, we just haven’t maybe been as good as in the first five games.
“We’ve got to understand as well that we’re coming away from home in the Championship, it is a different challenge, and we can’t shy away from that.
“I feel like we could’ve done a little bit more to put ourselves in a stronger position when we were leading, and until I see the replay properly back, I’ll hold back on the rest of it.”