OOO has broken my game because of major conflicts, and because i didn't Omod install it, i'm now having to do a full oblivion reinstall.

I've brought the Oblivion DLC's in the past, and installed them however i've deleted a couple of the DLC exe's by mistake, and now cannot find them. And i've been told that you cannot redownload them unless you rebuy them. Is there anyway I can actually get them back - i've got the receipt for them if that helps :/

Have you tried contacting Bethesda customer support?

Well, that's new.
OOO has very few conflicts with any mod apart from other game overhauls. For just about every other known conflict there's a link in the OOO thread for patches to make them compatible.
Unless you screwed the installation process it would make no difference whether you used the installer or OMOD version.