It is not possible to 'stretch' the O ring of a yoke system or DIN system. The valve annulus of the yoke system is designed such that the regulator rests against metal when fully seated. This is a designed in feature of valves made since 1977. The regulator must be screwed down until the connection is solid, much as when a valve is screwed into a tank. The most common problem with valves arises when clumsy removal of the O ring scratches the annulus. The entire valve must be replaced. If the valve is good replace the O ring with a white silicone type specifically designed for SCUBA valves. Buy it from a dive shop. If the regulator orifice is damaged, replace the nozzle. I don't know how these stories get started, eg that an O ring can be distorted by compression. Not technically possible. It is true, however, that old valves which used a thick O ring employed this ring as a crush washer. That is ancient history.