Dean Morgan's goal, a fine finish from Asa Hall's 57th-minute pass, ensured the hosts completed the double over the Dons and maintained their place in the League Two play-off positions.
"It was good possession attacking football from us and we didn't get a good bounce in their box," Brown said. "I thought we had a definite penalty in the first half that wasn't given.
"We won the midfield battle but Oxford won the battle of the boxes. We never really tested their keeper enough.
"How we didn't score a goal and didn't score a goal at Bristol Rovers last Saturday with all the play we had and all the openings we created beggars belief.
"But I don't think we're ruthless enough in or near the opposition penalty area and that's something we need to work on."
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