Published in 1989, this edition merged the original 12 volumes with four later supplements into a massive 20-volume set. Because this edition is still protected by copyright, you generally cannot download it as a free PDF. Instead, it is usually available via the Internet Archive’s Controlled Digital Lending (CDL) program, where you can "borrow" the digital book for an hour at a time.
Using the Internet Archive for OED PDFs comes with a few important considerations: oxford english dictionary pdf archive.org