Ashton, who spent a time on loan at Second division Notts County last season agreed to the move last Friday.
"I came down yesterday and trained with the lads," Ashton told the Oxford official website.
"I'm usually a centre back but I have played at full back as well. I hope I'll get a few games in the middle here though.
"I enjoyed my loan at Notts County last season because it was good to get away and play first team football.
"I hope I can have a good spell here as well and who knows what might happen after that."