The OED costs $100/year for US residents or £100/year for everyone else [0] but you can often get access through a library. The San Francisco Public Library has a proxy you can use if you have a library card there [1].
[0]: https://global.oup.com/academic/product/oxford-english-dicti...
Most people don't need the full OED and should probably just buy one of the smaller Oxford dictionaries. The Concise Oxford English Dictionary is an excellent single-volume dictionary of modern English based on the Oxford Dictionary of English (which is not the same as the Oxford English Dictionary. The OED is a historical dictionary, while the ODE is a dictionary of contemporary English).