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.

Setting -v OFMT='%f' (without having to embed it into my awk statement) worked for me in the case where all I wanted from awk was to sum columns of arbitrary floating point numbers.

As the OP found, awk produces exponential notation with very small numbers,

$ some_accounting_widget | awk '{sum+=$0} END{print sum+0}'
8.992e-07  # Not useful to me

Setting OFMT for fixed that, but also rounded too aggressively,

$ some_accounting_widget | awk -v OFMT='%f' '{sum+=$0} END{print sum+0}'
0.000001   # Oops. Rounded off too much. %f rounds to 6 decimal places by default.

Specifying the number of decimal places got me what I needed,

$ some_accounting_widget | awk -v OFMT='%.10f' '{sum+=$0} END{print sum+0}'
0.0000008992    # Perfect.