David Moyes has been chasing the Belgium international midfielder, but West Ham have faced difficulties in agreeing a fee with Lille for his signature.
They have though made a breakthrough and will pay the Ligue 1 club a fee of €35m plus €5m in add-ons for the 20-year-old.
Personal terms are now being discussed, but West Ham may need time in order to find an understanding with Onana.
West Ham and the player’s entourage are strongly not on the same wavelength regarding personal terms, according to Belgian journalist Sacha Tavolieri.
Onana’s camp have a very different idea of the personal terms their client should be taking home at West Ham than the Premier League side.
West Ham will now have to do the heavy lifting on a contract which pleases Onana and also fits within their budget.
The midfielder joined Lille last summer from Hamburg, who are set to bank a portion of the transfer fee via a sell-on clause.