The primary purpose of OTE is to give synonyms for the common everyday words of English: words with roughly the same meaning as the entry word (headword). Some words, especially those for animals, plants, and physical objects, do not have synonyms, so you will not find entries for gerbil or geranium. There is, however, an entry for squirrel because of the phrasal verb squirrel something away, which has synonyms such as save, put aside, and stash away.
The words selected as headwords are general words that nonspecialists are likely to want to look up. It is the job of a dictionary, not a thesaurus, to explain the meanings of unusual words, such as supererogatory, so such words do not get an alphabetical entry here. However, supererogatory is given as a synonym at entries for the more familiar words inessential, needless, and unnecessary. A thesaurus can thus lead the user from the familiar to the unfamiliar, improving their word power.