Philliskirk, 20 joins the Kassem Stadium club for the second time in his career having previously spent time with the club whilst with Chelsea were he made just one substitute appearance in a 0-0 draw at Burton Albion in August 2010.
Oxford manager Chris Wilder told oufc.co.uk: "We know all about Danny and what he can do. He's done very well for Sheffield United's reserve side.
"He knows the set-up here and what we ‘are trying to achieve and we're delighted that he's come back."
He is likely to make his debut at Gillingham this Saturday.