The attached patch enables versioning for libotp, and gives it a soname of libotp.so.0. I didn't commit it to the code directly, because I wondered if you had reasons for not giving the lib a ABI version number.
I saw that you added a manpage generated with help2man. By next release, I think that otpasswd will be pretty close to fully compliant with Debian Policy.
1000-sonames.patch