SAP BASIS (Business Application software Integrated Solution) should be your 1st choice. This space usually deals with a group of programs/applications and tools that act as interfaces between the database, OS, communication protocols, and completely different SAP products. The role of a BASIS consultant is to manage the entire SAP system landscape - including however not limited to the following:
1. OS administration & database administration
2. SAP client administration and maintenance of the system landscape
3. Planning and deploying of system upgrades
4. Management of SAP users, authorizations, and profiles
5. Administration of change/transport requests
Considering your experience in OS management and administration, the above should be suitable for you.