The striker, who has been capped four times by Scotland, will join the League One outfit until the end of the season.
"I'm delighted to be here and I'm looking forward to it," PA quotes Iwelumo as saying. "It's good to get out and play some football, that's my drive and that's what I want to do.
"I've got a job to do until the end of the season and I'm going to give it everything."
Oldham also signed 19-year-old Reading winger Jordan Obita on a one-month emergency loan on deadline day.