Somerville College's Andrew Warren, 56, and US professor Wyndham Lathem, 42, are accused of fatally stabbing Trenton Cornell-Duranleau in Mr Lathem's flat.
Chicago Police charged the pair with first-degree murder after they were returned to the city from California.
Mr Cornell-Duranleau, a hair stylist, was found with more than 40 stab wounds on 27 July.
Mr Warren was suspended from his university job as senior treasury assistant after the body was found.
A post-mortem examination showed Mr Cornell-Duranleau had the drug methamphetamine in his system.
Mr Lathem, who has been sacked as a microbiology professor at Northwestern University in Illinois, was in a personal relationship with the victim, police have said.
He plans to plead not guilty, according to his lawyer.
An Oxford University employee has been charged with murdering a 26-year-old in Chicago.