ON and ahn are very similar...

Not to UK (inner) ears, Helen. At least I don't think so (would be happy to be corrected).

ahn to a Brit is more like "Ann", dwelling on the "a" sound. The "ah" is like what we say at the doctor's.

The "arn" sound in "aren't" goes in a slightly more "o- in on" direction, veering towards "aw-" a bit. Difficult to distinguish from the first example, I suppose. Still a long way away from "on" though.

What we need are elocution diagrams and/or sound files.