Combining vowels are used to connect a prefix to a root word that begins with a consonant. However, "ologist" already has a vowel in the root word "-log" (from Greek "logos," meaning "word" or "study").
Therefore, we simply add the suffix "-ist" to the root word "log" to create "ologist," meaning "one who studies."
Examples:
* Zoology - the study of animals
* Biologist - one who studies living organisms
* Psychologist - one who studies the mind