Greetings,

I think its not particularly operating systems but it can be called sytem engineering because an os has a multiple more aspects. A system engineer can start with providing assistance in the Hardware and the Software domain, as per job expectations. Eventually, you can engage to learn and add more skills, like the OS, Network, Virtualization etc, that only help you further add value to your role, and to an organization.
Salary packages depends upon your capability, skills, the way you present yourself in the interview and the company you will be hired by.

For freshers placed in a very average company the salary  may vary from 3 to 6 lakhs per year but if you are good you may even be placed with a huge amount as a fresher.

All the best