I followed this lecture series from the placements point of view : https://www.youtube.com/watch?v=_TpOHMCODXo&list=PLDzeHZWIZsTr3nwuTegHLa2qlI81QweYG&index=1

Here are the lecture wise notes from this series : https://github.com/0708Suzzie/Operating_System-/tree/main/OS_codehelp_Notes

Here are the compiled notes from this series : https://github.com/0708Suzzie/Operating_System-/blob/main/OS_codehelp_Notes/All_Lectures_complied_notes_CodeHelp.pdf

If you feel like you need practice from university point of view, here are the quesiton papers for university level exams : https://github.com/0708Suzzie/Operating_System-/tree/main/Papers

In case you have an interview, you can check out this link : https://www.geeksforgeeks.org/commonly-asked-operating-systems-interview-questions/