One of the many vagaries of English pronunciation. Nostr is caught in the crossfire between ghost, host, most and post and cost, dost and lost.  The closest words are all pronounced with the short o, like coster, foster, roster and zoster (the only outlier being poster). I don’t think there’s a right answer but would lean towards the short o. What does nostr:npub180cvv07tjdrrgpa0j7j7tmnyl2yr6yr7l8j4s3evf6u64th6gkwsyjh6w6 say?