The one time I tried it, I did not like my experience with olive oil; and I swished for 20 minutes. When I spit the Extra-Virgin Olive-Oil out, it was yellow. I prefer sesame, walnut, or sunflower oil. Each of these 3 turn white, so I am sure they have done their work.
Did you swish your mouth with warm salt-water and then brush your teeth after your op?