I've found this guy "Bobby Parrish" to have some helpful videos on youtube. You might want to take a look at this one.
"You're Buying Fake Olive Oil...Here's How To Avoid It!" (Bobby Parrish) [8:02]
One of the brands he recommends is "California Olive Ranch - 100% California". (That company also make one using oils from several other countries, but he says don't get that one, just the 100% California one.)
He also recommends some other brands in that video.