HOW did researchers gather data for the Oxford study?

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.