During the installation of SCOM 2016, you can choose to deploy the Reporting Server feature. This feature will provide the possibility to generate SQL Server Reporting Services (SSRS) reports based on SCOM data. During the installation, it will automatically configure the Operations Manager Report Operator default SCOM user role as System User of your SSRS site.
You can check it by getting the Identity Id in SCOM console and cross checking it with the security configuration of your SSRS site.
And by default, this SSRS user is only granted to Browser, My Reports and Report Builder roles. However, if you want to provide members of this SCOM User role to create a scheduled report, you will need to add Content Manager role to the SSRS user as shown below.