The tone of Keats' poem "Ode On The Poets" is one of cheerful, optimistic enlightened enthusiasm. It's an uplifting poem because the poet is uplifted and inspired by his contemplation of the great Bards of the past, those who wrote of "Passion and of Mirth." You can tell this is the poetic speaker's tone of voice because he starts out with a triumphant proclamation that the great poets have left their souls on earth: they live yet and cast their spells yet. He follows it up with a question about their heavenly souls. In a paraphrase, Keats asks : If your souls are on Earth, are they in Heaven too? Do you have two souls for a double-lived spiritual life? The tone is then confirmed as he goes on to list one beautiful or lovely thing after another: they "commune" with "sun and moon" by "fountains wondrous" and "heaven's trees" on "Elysian lawns" with "blue-bells ... daisies ... rose" and a "nightingale" sings "melodious truth."
The structural style or form of the poem is that of an ode. Keats takes some liberties with the form and varies it for his own purposes. The first stanza is the strophe, with 22 verses (lines). It identifies the thing the ode is meant to praise: the great bards of the past. The second stanza is the antistrophe, which gives a turn to the subject that provides a balancing point of view to the praising strophe. In Keats' antistrophe the balancing effect can be seen is the mildly negative language Keats uses, which stands in contrast to the bright words of the strophe. He uses words and phrases that have subtle negativity to them, like "cloying," "little week," "sorrows," "spites," "shame," and "maim." The last four lines form the epode of the ode in which he completes the strophe and antistrophe and brings both together. The epode is actually a variation of a repetition of the four opening lines. He completes the ode by asserting the answer in the last four lines to the question he asked in the first four: Yes, the bards do have souls in heaven. Yes, they are "Double-lived" in heaven as on earth.
The rhyme scheme is an original one for an ode. The rhymes are couplets, none of which repeat a rhyme. This means that each couplet is assigned a different letter designation from a through r, which ends the 14 verse antistrophe. The rhyme scheme is aa bb cc dd ee ff ... oo pp qq rr. The exception to the pattern is the last four lines, which repeat the aa bb scheme of the opening lines. Once you have tone and structure figured out, the theme becomes evident: Keats, who is the poetic speaker in this poem, is celebrating and praising the bards of old who still lend beauty and strength to the world of its poets because their souls still inhabit this plane.