The U's nightmare start to the campaign continued after being put to the sword by Kenny Jackett's side at Fratton Park on Saturday.
The Blues were the better side throughout, with Jamal Lowe firing in a double, Gareth Evans on target and Rob Dickie netting an own goal.
Evans also missed a penalty when the game was a stalemate in the first half.
The loss kept Oxford rooted to the bottom of the table after three defeats from as many games, while Pompey moved up to second.
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Robinson revealedÂ that his U's players were feeling sorry for themselves after their defeat at Fratton Park.
And he's demanded that they get themselves out of the rut.
The manager told the Oxford Mail: '˜It's not good enough, but it is what it is.
'˜Right now I think we're wallowing in self-pity a little bit.
'˜We're worrying and focusing more on the lack of luck, which I'm not a massive believer in.
'˜Everything they seemed to hit in the second half went in. We folded a little bit and I have to take responsibility. It's my team.
'˜I feel sorry for the players, because they're trying their best.
'˜They're giving everything and are just maybe feeling sorry for themselves a little bit.
'˜We've got to get ourselves out of the rut.'