Keats's "Ode to a Nightingale" is most likely the most well-known and widely-read of his odes, and the critic Allen Tate once said that this ode "at least tries to say everything that a poet can say." The ode exhibits several of the themes that we associate with poems of the Romantic Period in English literature--transience of life, altered states of reality, nature and the natural world, mortality, and the power of poetry to transport the poet.
The ode begins, for example, with an indication that the poet's sense of reality may be altered when he tells us that "and a drowsy numbness pains My sense, as though of hemlock I had drunk, Or emptied some dull opiate to the drains. . . ." From the start, then, we have a poet who is perhaps observing life and nature through the filter of an altered mind.
When we get to lines 25 and following (third stanza), Keats discusses another pre-occupation of the Romantics, the transience of life. We are presented with images of decay, old age and death: "Where palsy shakes a few, sad, last gray hairs, Where youth grows pale, and spectre-thin, and dies." Keats is, unfortunately, accurately depicting the aging of men, who often become victims of palsy, lose their hair, and die.
But in stanza 4, the poetry is able to escape the ravages of old age and time by taking flight on the "viewless wings of Poesy." In this stanza and stanza 5, Keats essentially describes himself as not particularly fearful of death--"Now more than ever it seems rich to die, To cease upon the midnight with no pain. . . ." He notes, however, that, having died, he's no longer going to be able to hear the nightingale's song.
In stanzas 7 and 8, Nature's permanence, another Romantic theme, is clear when Keats says that "Thou was not born for death, immortal Bird!" The nightingale's voice that Keats hears was heard thousands of years ago in classical times and even the biblical era. Keats bids farewell to the nightingale, whose song has enabled him to escape the reality of his mortality and transported him into the natural world for awhile. Consistent with his altered state of mind, the poet cannot tell if he actually heard the nightingale's song or if the vision and song were just part of a dream.