When do we use 'a' or 'an' before words starting with 'o'?

When do we use 'a' or 'an' before words starting with 'o'?

We use 'a' before words starting with 'o' when the 'o' is pronounced with a consonant sound. We use 'an' before words starting with 'o' when the 'o' is pronounced with a vowel sound.