How can I make awk not use scientific notation when printing small values?

You should use the printf AWK statement. That way you can specify padding, precision, etc. In your case, the %f control letter seems the more appropriate.

I was not able to succeed with the OMFT variable.

It is actually OFMT (outputformat), so for example:

awk 'BEGIN{OFMT="%f";print 0.000015}'

will output:

0.000015

as opposed to:

awk 'BEGIN{print 0.000015}'

which output:

1.5e-05

GNU AWK manual says that if you want to be POSIX-compliant it should be floating-point conversion specification.