To be honest i don't like then as oblits/mutilators. They just look like fantasy chaos ogres with weird heads. Especially the obliterator guns just don't fit into
40k like they are now.
I have no problem with using ogres as base but they are lacking as
40k model like they are now.
If you want to go with those orges this is what i would do.
Tech them up. Add some additional armorplates and cables coming out of their skin. And attach some of those cables to their weapons in addition with some techy looking bits to avoid the fantasy look and feel.
Additionally i'd strap some more weapons to them. Either mutating/sticking out of their bodies or strap them onto their backs in a large pile to represent the ability to swap weapons of the oblits and mutilators.
Or if you still looking for alternative models and have some money to spare. HiTech miniatures makes some alternative obliterator and mutilaor models.
http://hitechminiatures.com/2/product/info/75 http://hitechminiatures.com/2/product/info/103 Those are the boxed sets but you can also get the models seperately.