Oxygen tank safety question

[Fr3nch]

What I would like to know is if its dangerous to leave the valve on a compressed Oxygen cylinder open when not using it. The cylinder is fitted with a regulator and a very small diameter line at 500 psi. The cylinder is properly restrained and is in a location in compliance with safety regulations.

Heres the full story if you are interested:

I do lab work for a research scientists here in colorado. I just started using some new equipment that uses a compressed Oxygen cylinder. After running my first full days worth of experiments I cleaned up the lab, but forgot to turn the valve on the O2 tank off. The tank is configured as mentioned earlier. The researcher who is in charge of the lab found that I left the O2 valve open and gave me a very aggressive speech on how that is a major safety violation. I have never had any compressed gas safety training, and the researcher only mentioned to turn off the O2 very briefly. I did a google search and found nothing except a government report on fires starting from friction caused by small leaks, but none of the safety precautions they listed to avoid the issue included turning the cylinder off when not in use. I've had the feeling that the researcher doesn't like me so I want to know if this is actually a safety issue, or if he is just finding things in my work to yell at me for. If I'm working with compressed non-flamable gas, should he be providing me with training?