coconut oil might be good...it has antifungal properties. The oil will benefit even if you think there's no space between teeth there has to be micro spaces there. Oregano oil might be way too strong for you-even diluted (a few drops in carrier oil) I find it really burns my mouth. But the stuff IS a potent germ fighter if you can stand it.
Yes Whole Foods or any other grocery which carries natural & organic foods (Trader Joe's etc.)
I still would say use sesame oil first.