"Original Aspect Ratio", which may mean black bars top and bottom (letterboxing) for ARs wider than the 1.78:1 of high-definition television screens. Many movies are 2.35:1 or 2.40:1 or 1.88:1 in theatrical presentation (other less common formats have been used), all of which will have black bars if shown OAR. It just means showing the entirety of each frame as shot by the director.
I don't believe that this thread was created for arguing the merits of presenting films in OAR on HDTV, but rather to give people who prefer them a way of giving each other a heads-up when they catch them. If you want to read such arguments, there's a ton of that back at the beginning of the thread.