How to root my Omnirom using Multirom? I have Omnirom installed on my Nexus 4 inside Multirom. How should I root my Omnirom then? Problem is that the OS on which Multirom was setup always gets stuck in a bootloop for a reason not known to me. So I use other ROMs by getting them installed on my Multirom. How should I root my Omnirom then?
Omnirom is already rooted, if anything you might have to install SU. 

as to the problem with your primary ROM on multirom, reflash the primary over the top of itself, then when complete install multirom again. As long as you didn't wipe data, your other ROMs should still be intact.....my primary ROM in multirom got glitchy, so I did what I recommended to you and its working perfect again