The Oktoberfest and altbier styles are indeed very similar, with a few subtle differences. Certainly the yeast is a bit different, although if memory serves, it is becoming acceptable(?) to just go ahead and use a lager yeast to brew an altbier (or am I wrong??). Beyond that, altbiers tend to be a little more bitter, with very subdued hop character. A little hop flavor is more appropriate in an Oktoberfest beer IMHO, whereas with altbier the malt clearly dominates and the hops are more limited to bitterness without much flavor beyond some unavoidable noble spiciness. Also, an altbier tends to be a little lower in original gravity than a fest beer, i.e., while both beers might be considered "session" styles, an altbier is generally the lighter of the two styles. There are exceptions to challenge all of these general tendencies though. And then there is also the sticke altbier that is greater in strength, and the newer Oktoberfest beer is more akin to a stronger yellow helles than to the amber Vienna and Munich malt based lagers of old. So, indeed, the lines between styles can be blurry. One thing is for certain -- these are all very tasty, malty, German style beers. In both styles, the malt is king, and noble hops play a supporting role, though still important. For instance, you wouldn't want to go anywhere near a C hop in these styles, it would just be wrong in my book as well as in the books of most other folks. Some of the great things in life are just meant to NOT be Americanized, and these German styles are prime examples IMHO. Just sayin'. But you are right. Just how much difference is there between styles, really. Place a few examples side by side with no labels, take sips of each, and declare which styles are which. Good luck with that. But they're all great.