Is there any character like that?

[ReneZ]

The frequency of aiin, ok, cho.. all point to a meaningful distinction between a and o.

I do agree with that, but I am not absolutely certain of it...

There is some overlap between the contexts of o, a and y, but the context cannot help in deciding which character is written, in case it is unclear.

a and o have sufficiently similar shapes to allow intermediate forms and uncertain cases. Since the writing of these characters is occasionally careless, one could begin to suspect that it does not even matter.
That ai is much more frequent than oi may be because it 'looks better' or is more natural to write. But of course we don't know.

y has a very distinctive tail, and is never confused with a or o (but sometimes with q or l).

On the other hand, there are quite a number of words ending in -o, in places where -y would be more expected.
Such an o can essentially always be replaced by a y, and result in a valid, more frequent word.

We cannot judge if they are equivalent, or this is a signficant difference. To be on the safe side, we need to assume they are not the same.