Thursday at the Canadian Olympic Trials at Foothills Athletic Park, Theisen erased any doubt with a personal-best 20-9/6.32m effort on her only long jump.
She then threw 141-6/43.14m in the javelin and ran an easy 2:16.07 in the 800 to lock up second place.
The Humboldt, Sask., native finished second overall with 6,393 points. Jessica Zelinka won with 6,599 points, breaking her own Canadian record in the process. Theisen only needed to finish in the top three of the Canadian Trials to qualify for the Olympics.
She is the sixth Oregon alum to qualify for London Olympics, joining Americans Ashton Eaton (decathlon), Becky Holliday (pole vault), Cyrus Hostetler (javelin) and Galen Rupp (10,000 meters), and Australia's Zoe Buckman (1,500 meters).