It is lonely. In these short days Without her praise I, too, grow less.
The Druids' tree Is far more wise In the mean ways Of wintriness,
Clings to the leaf, However dry, Will not deny By letting go.
Seasons are brief, Sorrow goes by; Hold on till May, This will be over.
This will be gone, Hill, field, and stream Ripple and gleam In supple gloss
Of shining sun, And waking seem A healing dream,— Oh yes, oh yes!
Loneliness opens "Oak" in the speaker's own voice: it's winter, the days are short, and without someone's praise the speaker feels diminished. "I, too, grow less" says it plainly. Then the speaker looks at an oak, the tree associated with the Druids, and sees a lesson in how it behaves in cold weather. It keeps its dead, dry leaves rather than dropping them, and the speaker treats that stubbornness as wisdom.
From there the poem turns into a kind of pep talk. Seasons are brief, sorrow passes, so hold on until May. The speaker seems to be talking to themselves as much as to anyone else. The short lines and tight rhymes feel like a chant meant to steady the nerves. In the last stanza the mood lifts into a bright picture of spring, with hills and streams gleaming in sunlight. The closing "Oh yes, oh yes!" sounds half like joy and half like a hope the speaker is working hard to believe.