The custom model data value of the oxygen item (The Oxygen Helmet) is 1, if you want to use custom model in the oxygen item, you can use custom model data 1 in the skull element

Here is pastebin link if you not want download a .sk file: Link

Now controls potions via custommodeldata

If you are using a resource pack on your server, you can change the appearance of the oxygen tube by setting the custommodeldata value of the potion to 1.

New version is:
https://pastebin.com/PZ8F4j5P