not aware of or not noticing something, esp. what is happening around you: She was often oblivious to the potential consequences of her actions.
The slang term zoned out could also be used
Spaced out, stoned, unaware, oblivious, unconscious, drugged, narcotized. Derived from the 70's phrase "lost in the ozone", which led to the related terms "ozoned", "zoned", and "zoned out". I got kinda zoned out listening to Green Day and forgot to finish my term paper.
SUPPLEMENT: In light of the comment, what about:
in a daze
out to lunch
without a clue
head in the clouds
walking on air
playing without a full deck
in lala land
spacey [no, that's just a term or synonym]
in his own world
It's not clear what you are looking for.