The 28-year-old was expected to join Oxford on a permanent contract earlier in the summer, but the transfer eventually fell through.
However, Barnett has now moved to the Kassam Stadium on a deal that is due to last 93 days.
"Tyrone is a proven goalscorer at this level," manager Michael Appleton told the club's official website. "He is a strong lad, has pace and is always going to cause defences problems.
"We are delighted that he is here and we are really looking forward to working with him."