Unfortunately, mechanical dogs were lured by the noises and now don't want to let Oscar off the tuck. You have to chase them off somehow.
Controlling the water cannon
Using the short ladder, get one level down and check out the water cannon. Zoom in on the lower chain and take the pomp plug out (move mouse down), then zoom in on the upper one and unlock the fire hose coil. Change the perspective a little bit to the right, zoom in on the coil and release some hose - it will automatically fit where it should.
Now zoom in on the wheel on the pump (above the fire hose), turn it (with mouse) as far right as possible, and click on the handle on the cannon.
Walk down from the truck and go back to the metro station. Walk down the stairs and enter the decontamination chamber (on the left). Close the door with the red button (on the right from the door).