It's 2 words. "come on".
Not "common" (which is its own word with a COMPLETELY different meaning). Not "cummon". Not "comon".
You can contract it: "C'mon". That's acceptable.
Come on.
See?
I'll do it again, in a sentence.
"Come on, let's go to the store!"
Thank you. You may now return to normal LJ Friday.
-Charlie
will detonate if he gets one more email from a supposedly college-educated officer with "comon" instead of "come on".