The decision sees England's Katherine Endacott take the silver medal after finishing fourth last Thursday.
Australia's Sally Pearson had finished first but was controversially disqualified for a false start.
Natasha Mayers of St Vincent and the Grenadines now wins Commonwealth gold, only the second ever for the country.
The bronze medal now goes to Delphine Atangana of Cameroun.
A statement from the Commonwealth Games Fe-de-ration (CGF) confirmed the result of the B sample.
It added that documents relating to the athlete had been referred to the International Association of Athletics Federations (IAAF) "for whatever subsequent action it might consider appropriate".
Osayomi's sample contained traces of banned stimulant methylhexaneamine, which has only recently been added to the World Anti-Doping Agency's prohibited list.
The 24-year-old sprinter requested the testing of her B sample after she was provisionally suspended on Monday, following the initial test.
She could now face a two-year ban.
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