"O wind,
If Winter comes, can spring be far behind?" Here the 'O wind' feres to-

Apostrophe:
An address to someone absent or something abstract as if the person or the thing were present.
It is often introduced by the exclamation "O".
- Shelly makes use of Apostrophe in "Ode to the West Wind" at least seven times.
Here is one of them:
"O wind,
If Winter comes, can spring be far behind?"

Source: An ABC of English Literature - Dr M Mofizar Rahman