On leaving target area aircraft received a hit from underneath with bomb doors open. Climbed to 15,000ft. Flt Eng reported port inner coolant temperature was getting high and gauge was flickering. At 16,000ft there was a large explosion from the engine. The port outer cooland gauge was flickering in the same way and had to be nursed all the way home. Flt Eng had oxygen failure leaving target and carried out his duties in a state of near collapse.