In the title Ode on a Grecian Urn the word on may indicate location as well as subject. Depicted on the artifact that this song seems to be about is the performance of a song, which is sounded to the spirit by unheard pipes that are directed to “play on.” Are the two songs distinguishable? For that matter, a pipe is also a wine-cask, thus urn-like; vessel and instrument were brought together by Master Ford: “I think I shall drink in pipe-wine first with him [viz. Falstaff]: I’ll make him dance.” Compare the ambiguity in a title like A Midsummer Night’s Dream. The White Knight alone could disentangle such threads.
In its second appearance on is made to rhyme unhappily with tone, nor is the other rhyme in the first part of the second stanza perfect, as if to make the point that the “sensual ear” is no judge. Indeed, unheard/endeared contains inconsonant pronunciations of ear. Consider further the whole sequence unheard/play on/endeared/no tone, which presents un-/on/en-/no as well as tone. Here positive en– contrasts with negative un-, while on is actually reversed to no and no tone reverses play on in both sense and word order. All this expresses the paradox of silent song, of poetry for the eye, or the mind’s eye. There may be more: adjusting no tone to not one (cf. atone < at one) we think of the disunity of the rhymes, and of the painted lover and his lass; ditties of no tone may even suggest dittoes of not one, since poem and urn only ever repeat themselves (Plato, Phaedrus 275d).
One on remains, in the last line: “ ‘Beauty is truth, truth beauty,’—that is all / Ye know on earth, and all ye need to know.” This concludes a stanza containing much word-play (intentional? no matter): Attic … attitude, brede (breed?), marble … overwrought (unmoved … impassioned?), weed (garment?), tease (arouse?), etc. In view of that I am encouraged to regard the urn as a thing made of earth, so that ye who “know on earth”—ye observers? ye figured?—know on the urn, an earthy paradise in a little compass. Further, “all ye need to know” suggests knead, what ye do to the earth called clay in order to prepare it for shaping into an urn: in order to know, you must work upon a mysterious identity of the sensual and the spiritual. And what if the last words can mean “all ye knead to no”? —turning the given thing into no thing, as music becomes silence: “Annihilating all that’s made / To a green thought in a green shade.”
 This assumes that the contemporary pronunciations of the paired words in question were as distinct as those later given in the first edition of the OED.
 It is unknown whether the poet put quotation marks around “Beauty … beauty.”