New hero skills for OOTM
 

What/where are they?

I couldn't see anything in the skill trees... There are some new ones in the skill tree if I start the arena campaign, but the blurb states there are new ones for OOTM as well. Are they hidden, or is it some sort of weird bug?

They're given as quest rewards.

There are also a number of skills in the skill trees - replacing ones that were slightly underpowered or less used. For example, warrior gets a skills that adds to all resistances and such :)