"O" sounding like a consonant?

Hello!

I get the rule for using "a" instead of "an" for words that begin with a vowel that sounds like a consonat (example: a unicorn). However, reading about this, I found the example of "a one-legged man". I get the consonantal properties of the "u" in "unicorn" (it sounds like "y"), but I have no idea why the "o" in "one-legged" would sound like a consonant.

How come?