Striker Rosenberg has largely been limited to appearances from the bench since he joined the Baggies on a free transfer from Werder Bremen during the summer.
However, despite his limited impact so far, Olsson believes that his fellow Swede will show just what he can do over the festive period.
"He's still settling in but he knows what it's about. He's been a professional player for 12 or 13 years," said Olsson. "He's disappointed not to be starting but the competition we have up front is our greatest strength.
"When he's come on a few times, especially at Newcastle, he's shown he's a good player. When he gets his chance he will do well. He's a friend of mine and I've told him this league, around Christmas and December, everyone will get their chance. He will take it when he gets that chance.
"It's a matter of us performing well and that can make it difficult for some players. He came during the final weeks of the summer transfer window and had no club during the summer so he had no proper pre-season.
"That prevented him from playing too much in the beginning and now we're playing well."
Rosenberg has yet to score from six outings for West Brom.