The Irishman will struggle to break into a high-flying Hammers team.
The 29-year old hasn’t featured for the Hammers since appearing against Astra Giurgiu in a Europa League qualifier in July but is now nearing full fitness after a calf issue.
DeAndre Yedlin – a deadline day arrival on loan from Spurs – has boosted the Black Cats’ options at right-back but Allardyce could draft in his former player for additional cover.
O’Brien has been a bit-part player at the Boleyn Ground for the majority of his time there and is not the type of signing to set tongues wagging in Sunderland.
The Black Cats are rooted to the bottom of the league following Saturday’s loss to West Brom and need a response against Newcastle next weekend if they are to avoid losing touch with those around them.