The Ambush Card allows the OL to attack with 3 creatures when a new area is reveiled. Does that apply when a new Level is reveiled by the Hero's going through a portal, not sure from the card?

Mordak said:

Ambush
Play immediately before the start of a hero's turn. Activate (move and attack with) up to 3 monsters of your choice.

There is no restriction to play it when a new area being revealed. If there was. then a new level would count, as each level is an area.

I usually find Ambush works best immediately after the OLs turn rather than when a new area is revealed. That way you get consecutive attacks and items like shields etc are unlikely to have been refreshed. Or when the party has considered itself safe from spawns and that diamond Beastman War party, or Lone Troll, that couldn't reach them suddenly attacks in their turn.
The difficulty is in still having 3 monsters worth attacking with!

hmmm good point, I usually use it when an area is reveiled ie a door being opened in vanilla, but I have some XP to spend in RTL and have bought 2 points of trap treachery so far, (love animate weapons), and thought about ambush next time, but you right getting a second attack from a pair of blood apes or something equally tasty (silver) would be nice, and easier to do than an attack as the level is reveiled.

There is also a treachery power card that does something similar every time a new area is revealed. Dire Warnings, I believe.

Yep, it's Dire Warnings .

Dire Warnings tho is only for a new area being reveiled, so thats ok, but in most dungeons you can not set up creatures close enough to the Hero's to really use the card, but ambush lets you attack in the hero's turn. So you can attack in your turn then attack again (3 creatures) in the Hero turn, might be the difference in a kill or not, before the hero can drink a potion or nip to town via a glyph.