Close to 100% nitrogen in an old tank?

[Chris H]

But cylinders don’t rust that badly under normal usage, isn’t it ? Are you saving money by doing this ?

(real question here)

After moving around a steel 104, full of tumbling media, while O2 cleaning/servicing them, the cost of a little bit of nitrogen to dry them without flash rust, is negligible. This isn’t about rust during usage, and the cylinders are never pressurized with nitrogen.

 

[Compressor]

Sorry a bit late as I was away from the PC.
There is not enough information to make a judgment on COD. Even lack of oxygen causing death can have many different sub-categories.
So I'm not for speculation.

To the original poster: my condolences to his family.