The 37-year-old hunk reportedly locked lips with the 'Vampire Diaries' actress, 25, at Zachary Levi's Nerd HQ party in San Diego, California, on July 25.
An onlooker claimed the pair couldn't keep their hands off one another and ''were making out for 20 minutes.''
While another added that they were dancing provocatively up against each other on the dance floor.
However, their closeness wasn't a complete surprise to friends as the pair - who were spotted holding hands in January - have known each other for while after being introduced through their mutual manager Scott Wexler.
The source explained: ''Nina has known Orlando for a few years now. She's been out with him in social settings with mutual friends. She's always thought he was an attractive man.''
Despite the obvious chemistry between the two, it seems they're not looking for anything serious as of yet and are just enjoying their friendship for the time being.
A source close to Nina said: ''She's single and having fun!''
Orlando split from his wife of three years Miranda Kerr - with whom he has three-year-old son Flynn - in October last year, while Nina went through two break-ups in 2013.