Depending on the size of the organization and the scope of the solution, I've seen the role description differ rather dramatically across multiple organizations/industries. It is usually a good idea to collaborate with your solution provider/ solution implementer to determine what is needed for your organization.
Outside of that, here are a few resources that can assist you in crafting your descriptions.
1. Start with a review of the Security Best practices documentation
https://community.hyland.com/gallery/items/75671-security-best-practices-onbase-foundation-ep4-modul...
2. Review the Inheriting OnBase guide. Many years ago a White Paper (and a course at Community Live) was created to help new Admins, it's a fantastic checklist of things to do and learn
https://community.hyland.com/gallery/items/53582-inheriting-onbase
https://community.hyland.com/gallery/items/55244-communitylive-2017---onbase-101-inheriting-onbase--...
3. Review the System Admin MRG, specifically the Section on "Maintenance Tasks"
https://community.hyland.com/gallery/items/75528-system-administration-onbase-foundation-ep4-module-...
4. Check out the Training Learning Path for a number of ideas on common training and responsibilities
https://training.hyland.com/paths/customer/l0004-system-administrator-onbase
