Before uninstalling OptimizeMember, there is a way to reset the roles and capabilities back to WordPress default.
Kindly go to OptimizeMember >> General Options >> Define Membership Levels & Packages and then scroll down near the bottom of that section until you see the grey "Reset Roles/Capabilities" button.
Click the button to restore user roles to the WordPress default roles.
As always, please take a full database backup prior to running that tool.
There are also some 3rd party plugins available at the WordPress repository that might help as well if you do not want to re-install OP Member.
You might want to try this plugin here:
In addition, there is also an option to remove every trace of OptimizeMember at your database which you might also want to try for this issue.
When OptimizeMember is active, go to OptimizeMember >> General Options >> Deactivation Safeguards and choose No ( upon deactivation, erase all data/options ).
Please note that choosing this option will erase all OptimizeMember data and there is no way to get that back unless you have a full backup of your WordPress site.