The 24-year-old British cruiserweight caught compatriot Geoffrey Cave with two solid right hands.
The bout followed Anthony Crolla's points defeat by Jorge Linares at Manchester Arena.
“Sorry to everyone that missed me fighting,” Okolie tweeted. “The good news is a Won by KO in 20 seconds!! Bad news is I'm back in the cage for now.”
London-based Cave, 33, has now lost all three of his professional fights.
Okolie competed at heavyweight and lost to Cuban Erislandy Savon at the last-16 stage at the Rio Olympics.
He hopes to become a world champion within four years and is scheduled to fight on the undercard of Ricky Burns' super-lightweight unification bout with Julius Indongo on 15 April