UCI is not a DO school.
Many decades ago, before it was UCI school of medicine, it was the California College of Osteopathic Physicians and Surgeons, the only DO school in the state.
During the "revolution" in the 60s, it was changed to UCI school of medicine, then Western and Touro opened up later.
Like it has been mentioned before, Western and UCI probably share some clinical rotations during 3rd year, and definitely during 4th year (since you essentially set up your own rotations for 4th year from Western). Additionally, I know quite a few COMP graduates who are in residency at UCI at the moment.
Hope that clarifies things for you.