Hand tight scba probe oring?

Which fill station do you have?

North Shore sells some orings, but I would double check measurements against what you already have before ordering. If you have a Chinese fill station, no guarantees they are using a standard size. On the North Shore site, you're looking for CGA 347 orings.

http://www.northshorecompressor.com/categories/Fittings/CGA-Fittings/