The novel and script writer joins the London-based studio as creative writer and producer.
As well as Doctor Who novels Nuclear Time and System Wipe, Smith has provided scripts for games Evacuation Earth, Return to Earth and The Mazes of Time.
"Mediatonic's unique style and edgy sense of humour really defines its games and I'm looking forward to taking these qualities even further as the company expands its scope across a range of platforms and titles," said Smith.
"With such a talented team and creative atmosphere it's a real thrill to be part of such an inspiring studio."
Mediatonic director of games Paul Croft added: "Compelling stories and humour are a big focus for us in the games we make and are core values of our creative philosophy. Oli is a fantastic writer and I'm delighted to have him join the team to further cement our reputation in this area."
Mediatonic develops for a wide variety of downloadable platforms, and has recently worked with Sega to develop a Flash-based Sonic the Hedgehog level creator.
The studio will soon release a Walking Dead Facebook game in collaboration with Telltale Games.