The former England Under 21 international is believed to have agreed a two-and-a-half year deal to become Avram Grant's third signing of the January transfer window.
Gary O'Neil joins West Ham United
Gary O'Neil has agreed to join West Ham United from Championship side Middlesbrough after he agreed terms with the Premier League basement club last night.
O'Neil's future at Middlesbrough has been the subject of conjecture for some time with the Teessiders thought to be willing to let the former Portsmouth man leave the Riverside.
West Ham have been keen to bolster their midfield options and had tried to sign Steve Sidwell earlier this month before they missed out to Fulham for the former Aston Villa man.
O'Neil, who can play in central midfield or wide on the right, is believed to have agreed a two-and-a-half year deal to move to Upton Park.
The former England Under 21 international will become the third signing of the January transfer window for Avram Grant, the Hammers manager, following the loan signings of Demba Ba and Wayne Bridge from Hoffenheim and Manchester City respectively.