I'm chasing down an oil leak and needed to remove the upstream O2 sensor connector and struggled with this task way too long. After trying to get connector released from its attachment to the lower water hose cover plate, I finally found an easier way. The upstream sensor connector tab needs to be pushed in to release it from the cover plate and it's very difficult to view this tab from the rear of the engine and underneath an exhaust manifold water hose. Using a mirror and hand bent tool to get at the connector tab it was a struggle to get a tool on the tab from behind the engine and I then found a way to get to the tab from the top side. See the attached photos for a small opening to view the release tab from above and a way to get a flat head screwdriver down to the release tab.
The upstream O2 sensor connector halves can be separated by unplugging the lower half from the right side wheel-well but the upper half is still attached to the cover plate and needs to be released by pushing on the connector tab. For anyone replacing the upstream O2 sensor you'll need to the release the connector from the cover plate.