Answer:
The OED has a number of different ways of collecting information of this sort:
running directed and voluntary reading programmes
analysing the content of published historical dictionaries, especially ones written to describe a variety of English
searching through large databases of historical and modern text
asking for and responding to contributions from the public
Nowadays most of the data that arrives at the OED‘s offices has been captured in electronic form. But handwritten letters still arrive from time to time, and their contents are stored in the dictionary’s quotation files.
