Oxford United are targeting Sunderland's Zak Johnson in a transfer move, after the defender had a successful loan spell with York City last season. The U's are looking to bolster their defense following relegation from the Championship, where they finished 22nd above only Leicester City and Sheffield Wednesday. Oxford lost 10 players at the end of their contracts last month and a further five names returned to their parent clubs following loan spells in OX4. This has left United with only two senior centre backs in Ciaron Brown and Michal Helik, 21-year-old James Golding a third option for Matt Bloomfield. Sunderland defender Johnson is apparently on the Yellows' radar after helping York to 13 clean sheets in his 25 league appearances and also managing one goal and two assists in the fifth tier. The 21-year-old's contract is set to expire at the end of the month. Johnson's loan spell with York City was a huge success, and he will now face competition from League One clubs looking to offer him an avenue up the EFL. The Black Cats will likely allow Johnson to depart the club this summer, given his struggle to break into Regis Le Bris' first team. Oxford United's interest in Johnson is a clear indication of their desire to strengthen their defense, which was a major weakness last season. With the new season approaching, the U's will be looking to make some key signings to ensure they can compete in League One. Johnson's experience with York City, where he played a key role in their title-winning campaign, makes him an attractive target for Oxford United.