As you probably know, Yog-Shototh can awaken if the Other Word deck is depleted. One of the Mythos cards reads: "The next time the clock strikes midnight, discard all Other World cards from play and do not replace them." By "out of play", do they mean (as I think) that they are returned to the bottom of the OW deck, or are they removed from the game altogether?

"Discard" means "put to the bottom of the deck" or "put into discard pile." I can't remember which way you're supposed to do it in ES.

Whenever you take something out of the game entirely, FFG (or at least, in the Arkham games) tends to say 'return it to the box'. I'm sure the mythos card you refer to means discard 'back to the bottom of the deck', and wouldn't apply to Yog's ability.

On the other hand, I have yet to play any game of ES (and I'd guess I've played around 60 or 70) or Omens (played 200-250) where I've ever gotten more than 6 Other Worlds in play in a single game (never mind failing all of them), so the odds of ever awakening Yog through his slumbering ability seem to be nil. He could use a redesign- the least effective slumbering ability of all of the AOs, IMHO. You could certainly play that particular mythos card so that the OWs go back to the box to up the ante with Yog, but he needs something else to pose any real threat in that regard. At the very least, I think you ought to gain a doom (or 2) for failing an OW adventure, in addition to taking the OW out of play entirely.

Yog-Sothoth is in bad need of a redesign, and I somewhat expect a promo version coming this year's Arkham Nights (or at least, I really really hope!) I think a doom token for each discard should definitely happen, but I also think he should replace the Otherworlds he eats so that there's still the temptation to try for more. There certainly needs to be something that thins the deck a little faster...