I also assume that the ability to take OA's generally comes with an active combat stance. Unless someone (or something) is being particularly sneaky (bluff, perception, monster knowledge come into play), I assume you can see the sword threatening you. That strike didn't just come from nowhere; the enemy had to be on your case and actively harassing you to take the OA. Similarly for aura's.
In fact, I generally assume that some vague sense of the monster's capabilities - insofar perceptible - should be actively communicated to PC's. If an elf is running around with bow, the bow is worth mentioning. It might be worth mentioning that he has two arrows nocked if he can shoot two at a time, but maybe he's just quick. The fact that the dragon's saliva drips from his fangs and starts burning holes in the planks of the ship is worth mentioning. The way the fringes of the wraiths shape move through the crates rather than around them is worth mentioning. Basically, I think in general there's likely to be a bucketload of little impressions that the heroes perceive but player's do not (automatically). Maybe they don't notice everthing - but if players never have a clue what they're fighting until it pulls it's tricks, something weirds going on. I think it's usually the other way around, actually - it's common for player's to persist using powers that aren't really believable (say, target reflex even though the enemy is really quick) just because the necessary communication hasn't been made. Or, they'll forget to keep a few squares free for the wizard to place his nasty effect. Sure, in-game the wizard probably does that several times a day - but out of game that's been last week and your life is full of other stuff to think about.
I think it's better to err on the side of too much information than too little - of course, not for stuff that actually has some reason to be hidden or secret.
Dramatic things like threatening reach - I'm sure there's a reason conceivable for not telling the players, but I've not yet been in that situation. The hydra's many snapping heads always paying attention to you and harassing you at every step isn't something I'd expect them to miss. That's kind of like failing the see the effects of polearm gamble - oh, whoops, how could I have guessed that running into his lance would hurt?