The gift is contingent on OHSU raising at least $500 million for cancer within two years.
The gift, announced Saturday by the hospital, is the third one from Knight and his wife, Penny, to OHSU. A 2008 gift of $100 million went to the hospital's Knight Cancer Institute.
The Knights announced the gift Friday night at a gala for the cancer institute.
"It is incumbent on every one of us to do what he or she can to keep the miracles coming," Knight said, in comments distributed by the hospital.
Knight Cancer Institute Director Dr. Brian Druker, who helped develop a drug that targets genetic defects in a particular cancer while leaving healthy cells unharmed, said the pledge will help kick-start solutions to the next major challenges in cancer research.
"This is a transformative moment," said OHSU President Joe Robertson in comments distributed by the hospital, "not just for OHSU and for Oregon, but for the people who matter most.
"This is about changing the lives of cancer patients around the world."
In September 2012, Knight pledged $125 million to establish a cardiovascular institute at the hospital that is expected to help in recruiting doctors and bridging the so-called translational gap between the data found in research and its application in a clinical setting.