Olejnik was the outstanding player on the unbeaten Australian Schoolboys tour to Britain in 1991, but has had a chequered career since, which has taken him around a number of clubs, including Manly, Penrith, North Sydney and Perth.
He played for his former Perth coach, Peter Mulholland, at Paris last season, scoring nine tries in 13 games before going home with an ankle injury.
"Our coach Shaun McRae knows him from his Australian days," Saints' chief executive David Howes said. "He's a player with pace and natural ball- skills."
Olejnik, who has agreed a two-year deal, is Saints' fourth capture of the winter, following the Oldham trio of Paul Atcheson, Brett Goldspink and Paul Davidson.Reuse content