Synonyms for On occasion:

adv.

now and then (adverb)
from time to time, occasionally, at times, once in a while, now and again.
off and on (adverb)
now and then.
once in a while (adverb)
once in a while.

adv

every now and then (adverb)
occasionally.
now and then (adverb)
at times, now and again.
once in a while (adverb)
now and then.

n.

on occasion (noun)
now and again, at times, occasionally, from time to time, now and then, once in a while.

Other synonyms:

Other relevant words:
occasionally.