The word of is a true exception to the phonograms and spelling rules! 

About 98% of English words have a spelling consistent with phonograms and spelling rules. This leaves about 2% of words like of that contain some sound or spelling that breaks the rules. Often these are words that came to English a long time ago, and their purpose is to perform a functional or grammatical role in English. They do not contribute to the meaning, and they can be difficult to define outside of a sentence that uses that word.

When teaching how to spell the word of, consider:

• It is the only common English word where F says the voiced sound /v/ instead of its usual unvoiced sound /f/. It may be helpful to remind your student that /f/ and /v/ are a voiced pair.
• The schwa or unstressed vowel sound is used when pronouncing the O in the word of.
• Because words like of, a, and the serve a grammatical role and do not contribute to the meaning, the vowel sounds in these words are often reduced to schwa.