Joshua will defend his WBA, IBF and WBO world heavyweight crowns at Tottenham Hotspur Stadium on 25 September.
Okolie, 28, will be part of the build-up, taking on mandatory challenger Dilan Prasovic from Montenegro.
It will be Okolie's first defence of his title after stopping Poland's Krzysztof Glowacki in March.
Liverpool's former super-middleweight world champion Callum Smith will make his debut at light-heavyweight against Dominican Lenin Castillo, while Ricky Hatton's son Campbell will meet Spain's Izan Dura.
