all depends on the what the intended usage is.
for general upkeep of the mouth/teeth/gums, 1-3 drops on the toothbrush is good.
For immune system items place drops under the tongue, hold for a few seconds, then mix with water, gargle if you can and swallow:
small items 3-4 drops, 3-4 times a day would be enough
medium/high level issues 5-6, for a flu more like 7 time would be good
It is also good to alternate this with grapefruit seed extract. Icky tasting stuff. And that one would be 5-6 drops in water for medium issues, and 9-10 for the big guns 3-5x a day.
Now, I have been told, and I believe it is true. When using oregano oil for a long course of time, continuously for several weeks, you begin to loose some since of taste. So there needs to be practiced the days/weeks of rest with the herb.