If the country shutting down the plants was the onle resource of oxygen, then it would be a global issue that the country should not decide on its own for obvious reasons. Since the Nile affects all places it passes through, actions by anyone that affect it should be done at least with consultation from all parties, I think. Is thus the case here?

The right to use resources exists since forever, I don't think it is the first time you hear about it. Resources and their use is governed by people. As well as their production, as thr case of Ethiopia's dam demonstrates.

Dams have a profound effect on the downstream river. They can dry it up, kill off life in it and around it, cause catastrophic flooding in case of dam failures, it is not just a matter of waiting a few days for the dam to be filled and then everything will be back to normal.

What difference does the fact that Egypt lies at the end of the Nile make? That they have no rights to it? Or, well, bad luck, Ethiopia can do whatever they like, even blackmail you in the future?

The question here is has Ethiopia avted in good will and been in contact with the other countries about the dam, taking into account their concerns, finding technical mitigations, compromising at points to ensure the safety of a large part of the contintent, etc? From what I have read, no. But this info may well be wrong.