Out of curiosity, I tried appending &per_page=100 as shown below and found Oman that way (2nd screenshot near the middle).
Note: victoria should work as well. I accidentally gave this example for the more recent duplicate of this question here rather than on that post: https://forum.inaturalist.org/t/taxon-does-not-show-up-in-seach-in-lists/68971
Appending URL arguments like this works, but… I’m not sure if it was intentional for the code to allow it so perhaps is not something that should be relied on. Besides which, it’s kind of hard to see amongst all those results.
So, having discovered that, I tried a slightly more useful variant of this that returns fewer results. I cut-and-pasted from the ray-finned fishes page the id# of the parent taxon of the taxon we’re looking for and appended it as a URL parameter &taxon_id=47178 and sure enough, this is accepted and narrows down the results to fish only:
Finally, I realized that the query terms could be left blank, allowing a direct search of the taxon (and all of its children) by id# by just typing &taxon_id=90450 (i.e. the id# for Oman).
Don’t celebrate yet, though! As I indicated above, if this is a bug and not a feature, this may or may not continue to work in future.