another word for chasm that begins with o
omission
opening
oversight
Another word for in front is, obverse.
originonset
Oasis
offence
order
obi
You might be thinking of the word 'omit' or 'omitted'.
One "wow"word (or phrase) beginning with O is "Oh my goodness!" Another = "Oh Wow!"
The word you are looking for is probably - legerdemain. Another synonym for sleight of hand is - Prestidigitation.
Ostrich!
sandcastle
Occult