O'Neil has been the subject of a number of inquiries since the transfer window opened at the start of the month, with the likes of Stoke, Blackburn and Wolves all having expressed an interest in his services.
However, West Ham leapt to the front of the queue when they tabled a formal £1.5m bid yesterday, and the midfielder will conduct a medical and sign a two-and-a-half year contract in east London later today.
Having made what is almost certain to be his final appearance as a Boro player on Saturday, he could even be involved in tomorrow's Carling Cup semi-final second-leg tie between West Ham and Birmingham. O'Neil was not involved in either of Boro's two games in the competition this season, and is therefore not cup-tied.
The 27-year-old was the subject of interest from West Ham earlier in the transfer window, but at that stage Boro rejected the Hammers offer of a loan move.
As a result, West Ham boss Avram Grant explored the possibility of signing Steve Sidwell instead, but that move collapsed when the former Aston Villa midfielder signed a short-term deal with Fulham.
O'Neil became the Hammers' preferred option again, and the weekend dismissal of Frederic Piquionne, whose subsequent suspension has severely reduced Grant's options for tomorrow's game, has effectively forced the London club's hand.
While a £1.5m fee will represent a £3.5m loss on the sum Boro paid to recruit O'Neil from Portsmouth in August 2007, the Londoner's departure will result in a significant reduction in the wage bill.
O'Neil is one of the club's highest-paid players and is understood to earn around £35,000-a-week, a figure that will not be sustainable in the absence of a full Premier League parachute payment next season.
His departure will leave a considerable hole in the Boro midfield, however, with Kevin Thomson ruled out of the majority of the rest of the season and Barry Robson facing a lengthy lay off after injuring his ankle in Saturday's 1-1 draw with Preston.
O'Neil has featured in 17 of Boro's 27 Championship matches this season and made a total of 114 starts during his time on Teesside.
Tony Mowbray will be given permission to replace him, and having anticipated the midfielder's possible departure all month, the Boro boss has already compiled a list of possible midfield replacements.
Robson's injury has increased the need for movement ahead of next Monday's transfer deadline. The Scotland international suffered an ankle injury during a first-half collision with Preston full-back Billy Jones, and received lengthy treatment during the half-time interval on Saturday.
"Barry had a gaping hole in his foot just below his ankle, obviously created by a stud," said Mowbray. "It wasn't very pleasant to see at half-time. I don't know how many stitches he's had in there, but obviously he was unable to continue."
Willo Flood is still more than a fortnight away from a first-team return, while fellow long-term absentee Rhys Williams is unlikely to return to match fitness for at least another month.
In the wake of Wheater's departure, Mowbray has expressed an interest in Maximilian Haas, a 25-year-old centre-half who is captain of Bayern Munich's second team.
Haas is out of contract in the summer, but Jonathan Grounds' return from a loan spell at Hibernian has reduced Mowbray's need for a central defensive reinforcement.
* Gordon McQueen has left his senior scouting position at Rockliffe Park. The former Manchester United and Leeds defender had scouted for the club for several years, having previously worked as a coach during Bryan Robson's managerial reign.
* Former Middlesbrough striker Massimo Maccarone has joined Sampdoria after scoring just two goals in 18 appearances for his former club Palermo.
Source: Northern Echo
Source: Northern Echo