If a writer uses onomatopoeia, he or she (usually a poet) uses words that mimic or echo the actual thing described. For example, the word "run" has nothing to do with the sound you make when you're running, but "buzz" ends with a buzzing sound, and so would be considered onomatopoeia.
In "To Autumn," Keats uses fairly subtle versions of onomatopoeia. I'd say that the vowels in "oozings" late in the second stanza sound like something, well, oozing. When "the redbreast whistles" in the third stanza, that verb sounds like some forms of whistling. In the final line, "twitter" sounds like what birds sound like when twittering, and so would be onomatopoeia. Keats' "wailful choir" in that same stanza works: the word "wail" sounds like someone wailing.
Examples beyond these get subtle indeed, and open to more interpretation. When Keats says hair is "soft-lifted by the winnowing wind," you can hear the air moving in "lifted," but you have to connect wind with lift for that to really work.