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. 

 IMO it's just pronouncing it the way it's spelled, so either "no" or "nah" makes sense to me. Doesn't really matter if the "n" stands for notes. 

 Tell that to the Gif and Jif crowd.

(It’s obviously GIF by the way 😆)