You can strip it off the front using parameter substitution like this:
a="(STDIN)= 0A0A9F2A6772942557AB5355D76AF442F8F65E01"
b=${a#*= } # Strip up to "=" sign
echo $b
0A0A9F2A6772942557AB5355D76AF442F8F65E01
I suspect that there is an option to openssl to stop it generating the "(STDIN)" part, but mine doesn't do that.
If you want to learn more about this sort of thing, have a look here.