Turner starred as young soldier Molly Dawes in the BBC drama pilot, which aired in March 2013.
A full series was commissioned in December, with Game of Thrones and Misfits star Rheon now joining the cast.
Ben Aldridge (Lark Rise to Candleford), Charley Palmer Merkell (Mayday), Arinze Kene (EastEnders) and Ade Oyefeso (Youngers) will also feature in the five-part run.
Turner has also recently reprised her role of Stacey Slater on EastEnders after a break of over three years.
Our Girl is expected to air on BBC One later in 2014.
Iwan Rheon: 'I'd be an idiot not to do a Misfits movie'