Operating system resources:
1.http://pages.cs.wisc.edu/~remzi/OSTEP/ [best intuitive book on OS]
A companion video series for above book by prof Mythili of IITB: https://www.cse.iitb.ac.in/~mythili/os/
2.https://www.cse.iitb.ac.in/~mythili/teaching/cs347_autumn2016/ [ it contains in-class materials and programming assignments ]
3. CS - 347 IITB
I hope these will be useful for those who are still in UG college and have started to learn OS. For Gate Preparation the videos will be a nice revision of basic concepts.