I love to rise in a summer morn
When the birds sing on every tree
The distant huntsman winds his horn
And the sky-lark sings with me
O! what sweet company
But to go to school in a summer morn
O! it drives all joy away
Under a cruel eye outworn
The exclamatory ‘O’ is a common device used by Romantic poets to achieve emphasis. 
 Note that this line is  catalectic  or shortened in terms of metre. It suggests the emphatic joy of the schoolboy. 
 Note the word ‘company’; the child feels he has a special personal relationship with wild creatures; they are imaginary friends.
The Schoolboy