Can’t remember for sure but on my plain vanilla Debian 9 install, I think conf files are in /etc not /opt
Sorry not more help but away from computer at the moment. Search for it
There are two different ways to install OH2 via the repository or manually. The manual way will put files under /opt/ the repository way will put them under /etc/; from the looks of it, you installed via the repository way and you will have no files under /opt/ by design
see here for help on file locations: https://www.openhab.org/docs/installation/linux.html#file-locations