They all work the same. You just have to be careful to make sure you read it right; a positive is a test line that's as dark as or darker than the control line, and almost as dark doesn't mean you're about to get a positive, as LH (the hormone OPKs detect) goes up and down throughout your cycle anyway. The easiest thing is to get one of the digital tests that gives you a smiley face when positive; it eliminates the guesswork.
HOWEVER, a positive OPK does NOT guarantee that you will ovulate. It's entirely possible for your body to prepare to ovulate without actually releasing an egg for whatever reason. The best thing to do is chart your cycle (especially basal body temp and cervical mucus); CM will tell you the same thing OPKs do, but for free; and BBT will tell you for sure whether you've ovulated.
at 11:04 PM on Feb. 12, 2011