Regarding Mahakaleshwar darshan prior booking is a must. You can do the bookings online and offline, though offline might take some more time.
Omkareshwar has both bookings you can check on the temple website, else in temple premises you can make the bookings. Belief is during weekdays its easy to see the Lord at the earliest.
For Mahakaleshwar if you looking to attain Bhasma Arti. thn book in advance online booking weblink is www.shrimahakaleshwar.com
For omkareshwar also have online booking facility.