The Estonian international forward, who can also play in the wide positions, linked up with the squad earlier today but is ineligible to face Crewe this weekend as the club are still awaiting international clearance.
After starting his career at Derby County, Ojamaa went on to play for clubs abroad before moving back to the UK and plying his trade with Motherwell in the Scottish top flight. For the past two years, he has been in the Polish Ekstraklasa with Legia Warsaw and has played in European competition for the Polish side.
Last term, he spent time on loan at Motherwell and at Norwegian side, Sarpsborg 08.
An international at junior level for Estonia, the frontman has also earned senior honours for his country.
Henrik will wear the number 32 during his stint at the County Ground.