The Premier League new boys are working on capturing the Huddersfield man, but their pursuit has been complicated by the fact he has two exit prices.
One clause in his contract has been set at £10m, however in a twist, a newly promoted club must pay a higher sum to sign him.
Nottingham Forest are close to matching the lower figure and do not want to pay the higher figure.
The two clubs are in talks, but despite the twist, the deal is still expected to go through.
It is suggested the deal will be sorted out within the next day or two, with O’Brien on course to join Nottingham Forest.
The Tricky Trees saw O’Brien up close in the Championship last season and boss Steve Cooper was impressed with the player’s performances.
Leeds United wanted O’Brien last summer, but were not prepared to pay the fee Huddersfield were demanding.
