In PeopleSoft you have persons (identified by EMPLID) as transactional data, and users (identified by OPRID) who can access the application.
Not every user is necessarily related to a person or employee (e.g., system accounts such as PTWEBSERVER, developers' accounts, etc.), and not every person or employee will have a user profile (e.g., employees without access to Self Service transactions, ex-employees who no longer work at the company, etc.).
So, to answer your questions:
Yes, OPRID is only related to user profiles (you can obviously find OPRID values used elsewehere, like in "Last Updated By" fields, security tables, etc., but it only relates to users). Often an OPRID will be linked to an EMPLID (in the PSOPRALIAS and/or PSOPRDEFN tables), but this is neither guaranteed not required.
Generating a user profile for every employee is not always warranted, as there are important security implications to it. Furthermore, if your company uses single sign-on with LDAP authentication, the users are automatically created upon first sign-on if so authorized, so proactively creating them is unnecessary and often ill-advised. Finally, it will often be a security breach to have ex-employees with active user profiles, so you may never truly reach a scenario where all your EMPLIDs have associated OPRIDs. If you've weighed all of these concerns and do indeed want to pre-populate the entire user population, you can use something like Excel-to-CI. However, note that this will solve the issue as of right now, and the moment you create a new employee you'd need to also create its user profile.