Words that only the vowels I and O:
Dominion
Idiot
Logic
Here's an explanation as to why these words wouldn't fit the above category:
Courtiers - No, this uses the vowels I and O, but it also uses the vowel e.
Crockery - No, this uses the vowels I and O, but it also uses the vowel e.
Dangerous - No, this uses the vowels I and O, but it also uses the vowels a and e.
Disposal - No, this uses the vowels I and O, but it also uses the vowel a.
Nonsense - No, this uses the vowels I and O, but it also uses the vowel e.
Poisonous - No, this uses the vowels I and O, but it also uses the vowel u.
Premises - No, this uses the vowels I and O, but it also uses the vowel e.
Probable - No, this uses the vowels I and O, but it also uses the vowel a.
Sensible - No, this uses the vowels I and O, but it also uses the vowel e.
Please Note: Bold letters are vowels.
Hope this helps! :)