For a long time I thought that ISO was pronounced as the separate letters of the word; that is, "I-S-O." (I think that for me, this was a carryover from, "ASA," which was spoken as the three separate initials of the American Standards Association. Showing my age here a bit.)
Then, I heard others pronouncing it as, "EYE-so." That didn't seem right to me, so I started looking around the web and found conflicting advice.
Some were suggesting that ISO was an acronym from the name of the organisation — the International Standards Organization. A nice theory, except that their name is actually the International Organization for Standardization — those initials would be IOS.
To resolve the issue I devised a cunning scheme. I went to the ISO videos produced by the organisation itself to see how they pronounced it:
"EYE-so," it is.
Turns out I didn't need a cunning scheme. The ISO media page says this about their name:
From the Greek isos, meaning equal. Whatever the country, whatever the language, the short form of our name is always ISO.