The 35-year-old had been out of contract after leaving Stoke at the end of last season. The striker had a trial at Hull City in the summer but failed to earn himself a deal on Humberside.
"It went quickly. Only a few days ago I got in touch with the manager Kenny Jackett," Oemwingie said via Rotherham's official website.
"Knowing him, he is a manager that believes in you and that gives me a lot of positivity. He obviously believes in me and thinks I can do a job for him, so I am looking forward to a new start and doing my best for the team".
The Nigerian had a spell in the Championship last season with Bristol City where he scored two goals in seven appearances for the Robins.