Reports in Turkey suggest that a £4m fee has been agreed for the 24-year-old, who is due to fly to the Midlands to undergo a medical.
The Blues have reportedly also made a £7m bid for Torino midfielder Afriyie Acquah, but Redknapp has suggested that he will only sign one of the two.
"You've got to see what you can get really," Redknapp told Sky Sports News.
"If you can get them both you've got to make a decision, if you can only get one you take him, but if you can't get any, you just pray, I don't know where you go... but we're trying to get them in."
Onazi, a Nigeria international, joined the Turkish club from Lazio last summer and made 32 league appearances, scoring two goals.