these are callled offside if the player in an offside position is in the view of the keeper, or if the player could still impact the play actively and therefore influence the keepers decision to go after the ball or not.

in this case it looks to me like onana has a clear view to the ball and the player is also not in a position to have an impact on the play and go after the ball in any other way.

by the rules this is the correct call (if he wasnt in his view - maybe the angle is deceiving and he is in his view?)