The 24-year-old joined the second tier team last summer but struggled for playing time after Dennis van Wijk, the manager who signed him, was replaced with ex-Leicester City boss Nigel Pearson.
Watt announced his free agency through his Twitter feed on Monday afternoon.
He wrote: “One chapter closed. Another one opens. Thank you @ohl_official for the time I spent! Unfortunately I never played much football when the new manager came in but we left it on good terms.
“That’s football. I’ll have time to see my next step now!”
He also added: “I’m predicting some negativity so I’ll delete the Twitter app for 48 hours troops have a good one! Back to [Scotland] now.”
Watt came through the ranks with Airdrie United before moving to Celtic in 2011. He famously scored the second goal in their 2-1 victory over Barcelona a year later, but it wouldn’t be long before he left the club, signing for Standard Liege after impressing in a loan spell at Lierse.
In 2015 he was back in British football, joining Charlton Athletic. During his time at The Valley, he undertook loan spells at Cardiff City, Blackburn Rovers and Hearts, before moving back to Belgium in 2017.