Nos-tr sounds better, but doesn’t lend itself to the ostrich connection.

Nah-str gets the ostrich connection but makes absolutely no sense.

If we went with the “o” sound from “notes” then it’s No-str. 
If we went with the “o” sound from “other”, then it’s Nuh-str.
If we went with the “o” sound from “nostril” then it makes no sense whatsoever and why on earth would we do that? But we do get ostriches.

I literally say it both ways all the time.