I stopped using toasted sesame oil and switched to cold-pressed grapeseed oil (which I always have) with 1-2 drops of oregano oil.
Yesterday I bought cold-pressed extra virgin coconut (had a hard time finding it) and started to OP with it last night.
I have been doing OP twice a day. The yellow stain from the toasted sesame oil hasn't come off yet, but I think my teeth is getting somewhat whiter (can't be too sure about this since the result isn't obvious enough). Hope the yellow stain will come off in a week or two.
(The label of the sesame oil that I unfortunately used says "Pure Sesame Seed Oil". I hope there is no added color to it. Otherwise, I may have real difficulty in getting rid of the yellow stain.)