What is a word?

Dictionaries  struggle to precisely define the nature of their core commodity.
Cambridge defines a word as a single unit of language that has meaning and can be spoken or written. Collins agrees but adds, In English, a word has a space on either side of it when it is written.

Brittanica includes another dimension: a sound or combination of sounds that has a meaning. 

This pointy-headed pedantry does have practical ramifications. We may know what a word is, but Bill Clinton showed it is possible to  haggle over the exact the meaning of the word is.

There are also circumstances in which a word count is essential, as journalists and essay writing students well know. And to count words we need exact criteria as to what we understand them to be.

How does a linguist define a word?

The OED distinguishes 430 senses for the verb set. The dictionary essentially considers these homographs as separate words but there are complex rules about this - see here

Linguists make the distinction between headwords and lexemes. The headword ‘run’, for example, includes all its grammatical forms: run, running, ran etc. 

Are all the words in OED in common use?

* 41,700 OED words are obsolete. This means that you are very unlikely to use them in either speech or writing.
* 240 are ghost words. A ghost word has never existed outside dictionaries.