Yup, buy from Amazon.com directly and you're ok. If from 3rd party sellers on Amazon, expect a fake and be pleasantly surprised if they're real. There are sites that point out the differences between fake vs. real Oral-B brushheads, as they've gotten quite good at making them.
Some say as long as it works the same, who cares. But then you have to consider that counterfeiters don't care about your health. There could be lead in the fake brushheads that you're sticking in your mouth every day. Not to mention the bristles could be doing more harm to your gums than good. And with brushhead changes every 3-4 months, you're better off saving a few dollars a year on other areas that don't affect your health.
I am not so sure about buying from Amazon will entirely clear that up.
There was a reviewer who said the brushes he bought were fulfilled from Amazon.
http://www.amazon.com/Oral-Precision...DateDescending
"totally crappy. Fake Brush Head. Fulfilled by Amazon and it STILL was a FAKE., March 22, 2011
Amazon Verified Purchase
This review is from: Oral B Precision Clean 3 pack brush head refill (Health and Beauty)
I only purchased these because the order was fulfilled by Amazon - I had seen the warnings about knockoffs being sold, and that was supposedly how to ensure you were getting the real deal. It didn't matter, THEY WERE STILL FAKE. Don't waste your time, pay full price in a store where you know what you're getting. Never again. "
Another poster mention ed they were fakes that were labeled From Germany:
http://www.amazon.com/Oral-B-Sensiti...DateDescending
"Buyer Beware, Check your replacement brushes!, August 8, 2011
Amazon Verified Purchase
This review is from: Oral-B Dual Clean Replacement Brush Head (3 ct.) (Health and Beauty)
I received a fake set of replacement brushes, pretty disappointed. however, i did get my refund, But I am lost as to how to get a good price on a genuine set of brush heads.
I knew it was fake because packaging seemed cheap, made in Germany, bristles looked different; like a regular tooth brush, genuine brush heads are more compact.
I just want genuine replacement brushes. "
Yet another poster/reviewer said thsis about the product being labeled From Germany:
http://www.amazon.com/Oral-B-Sensiti...DateDescending
"5.0 out of 5 stars Genuine product from German., December 8, 2011
Amazon Verified Purchase(What's this?)
This review is from: Oral-B Sensitive (3 Extra-Soft Brush heads) (Health and Beauty)
This is the second time I bought these from Amazon for my Braun elec toothbrush. Genuine product from German. Works like a charm. I've always prefer soft brushes since my teeth are sensitive."
LOL. Now I don't know what to believe but jrichrds is probably right in that purchasing only from Amazon itself may increase your chances of getting what you paid for.