Ms O’Keeffe stepped back as mayor when she ran for the state seat of Shepparton and, at the time, said she would resign from council if she was elected to Spring St.
She won the seat ahead of sitting member Suzanna Sheed and Liberal candidate Cheryl Hammer.
Ms O’Keeffe spent four years as mayor in her time on council.
The vacancy will be filled through a countback of all the votes from the last general election held in October 2020.
It will include all remaining eligible candidates who were not elected at the general election.
It’s the second countback for this term of council, with former councillor and deputy mayor Rob Priestly stepping back after his unsuccessful tilt at the federal seat of Nicholls in May.
Mr Priestly intended to return to council, but said he had developed too many conflicts of interest during his run for federal politics.
His seat was filled by countback in July, with former mayor Dinny Adem returned to council following the process.
All votes from the 2020 council election will be included in the countback, with successful candidates required to achieve a quota, which is the same process used in regular council elections.
Before being elected, a successful candidate must complete and return a declaration stating they are still qualified to be a councillor.
If the candidate does not return the declaration within 48 hours, another countback will be held.
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