The midfielder had been a free agent ever since his release by Birmingham City at the end of last season.
However, he has now agreed a contract with Oxford that will run until January, with the option of having that deal extended until the end of the campaign.
"Brian is a very experienced player and he has that bit of quality and know-how that you need. He has been training with us for a couple of weeks and has done well," manager Michael Appleton told the club's official website.
"He knows the lads now and will go straight into the squad for the two home games this week against Accrington and Stevenage."
Howard, 31, has played for host of clubs during his career, including Barnsley, Sheffield United and Reading.