Like Pat said, even if both parents are clear, OFA will not consider the offspring to be clear. With the current test, the accuracy is not high enough for them to safely allow you to assume that. When the full direct gene test comes out in the future, you will be allowed to officially consider a clear to clear breeding as clear offspring too. However, even still, OFA will not include that info on the offsprings' pages. You still have to do your own research on the parents (or further back) and come to your own conclusions. The only way for any test results to ever show up on your dogs' pages in OFA for any disease, even if certain, is to have the test done on that individual dog too. Even if parents already prove what the offspring will be.
This is especially important if you have a dog from a pet store, backyard breeder, or other less-than-reputable person. There are quite a few cases of the dog not really coming from who the pedigree says it's from and not having the parents it looks like it has. You still want to test each dog to make sure what you see is what you get!