I've been playing around with Core on my home machine, and have extended the installation document just a bit. Take a look here: https://bitbucket.org/yminsky/ocaml-core/wiki/InstallationHints
I just given an example of a working _tags file for ocamlbuild and a .ocamlinit file for working in the toplevel. Also, use oasis-0.3.0-rc2 if you want to get working native-code builds.
We're also closing in on a proper release, with tarballs and everything. The tarballs should be considerably easier to install, and in particular should not require a separate install of Oasis.