The show, which stars James Nesbitt and Sarah Parish, was renewed by the broadcaster last July.
Oberman has now signed up to play the new nurse specialist Lizzie Clapham, who looks after the patients' emotional care.
Drop the Dead Donkey star Neil Pearson has also joined the cast as head of Clinical Services and Monroe's manager Alistair Gillespie, while Five Daughters actress Lisa Millet will play "acerbic" nurse Jill McHeath.
The new series of Monroe is expected to pick up 18 months after the first, with Monroe facing challenges from his ex-wife and son.
Bremner and Shepherd (Tom Riley) are dealing with an "unplanned development" which could affect their careers and their relationship, while Monroe's trainees Daniel (Luke Allen-Gale) and Kitty (Michelle Asante) are fighting for the role of registrar.
The first episode will also feature guest stars Jody Latham and Julia Haworth as Paul and Julie, who turn to Monroe for help after Paul is refused surgery for a dangerous neurological condition.
The show's producers Michele Buck and Damien Timmer said: "We were thrilled with the success of Monroe series one, and Peter [Bowker]'s scripts for this new run deliver yet more drama, wit and emotion, and we're opening up St Matthew's hospital in all sorts of ways.
"Our brilliant ensemble is even stronger than before, and at the centre is James Nesbitt's extraordinary performance as Gabriel Monroe."
Filming for Monroe is currently taking place in Leeds.
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