Oso Loop is named after Oso Mountain (the highest point in BBRSP). It's predominantly a jeep road with several shorter climbs, lots of loose rocks, and loose rocks in the climbs. It will however reward you with some amazing views of the western portion of the park. It ends at the Agua Adentro pens where it joins the west portion of the Main Park Road.
Side trips include a hike up Oso Mountain, or heading down Las Burras Trail or Rancherias Trail.