it's actually pronounced woo-ster in the U.K.
Are you saying that you don't pronounce the "shire" part?
I pronounce it "wooster-shur," which is how most Americans I know say it.
And as for taco, as mathanxiety pointed out the American pronunciation is based on the Mexican Spanish pronunciation. It's a Mexican food after all. Spanish is the second language that most Americans are most familiar with, but it is various versions of Latin American Spanish.
And I think oREGano is closer to the Italian pronunciation (of origano) than OreGAHno. Oregano is actually a Spanish word and the accent is on the second syllable as in the American pronunciation.