O'Callaghan, best known for playing Sergeant Boyden in the ITV police drama, has been cast as a new love interest for Jean Slater.
The storyline kicks off when Jean (Gillian Wright) is finally given her own allotment after months of trying. Jean is delighted to have somewhere she can regularly visit for some peace and tranquillity, but her quiet place is soon tarnished when she meets fellow gardening enthusiast Ollie Walters.
Although Jean and Ollie get off to a rocky start, they later become close. However, it's unclear whether the relationship will provide a happy future for Jean or turn out to be a disaster.
Speaking of his role as Ollie, O'Callaghan commented: "I am very excited to be joining a show with as much prestige as EastEnders and I'm looking forward to portraying the nice side and humour of Ollie."
Wyatt, meanwhile, will feature as a new character named Betty in a storyline with Patrick Trueman (Rudolph Walker) and Cora Cross (Ann Mitchell).
Betty takes over as Patrick's dance partner next month when Cora is distracted by problems within the Branning family.
Patrick gets on extremely well with Betty, but when Cora takes a disliking to her, he is forced to choose between the two women in his life.
EastEnders airs Betty's first scenes on Friday, July 5. Ollie makes his arrival on Thursday, July 11.
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