Newcastle striker Joelinton had two goals correctly disallowed for offside.
"I'm frustrated, disappointed, upset," said Bruce. "From the first minute to the last we were never good enough in possession.
"We gave the ball away. Our whole demeanour wasn't right. Oxford made it difficult for us. But we made it difficult for ourselves the way we surrendered possession. The amount of mistakes we made was ridiculous."
It was a celebratory atmosphere to start with - a 52,000 sell-out as rumours circulated about a possible Saudi Arabian takeover.
"The big crowd played their part today and we haven't done enough to make them enjoy it," said Bruce. "We huffed and puffed all day and didn't do enough."