Why does the $-OH$ group activate the benzene ring and direct substituents to ortho/para positions?

The lone pairs on the O atom of $-OH$ are donated into the ring by resonance, creating negative charge at ortho and para positions:

$$Resonance structures increase electron density at o- and p-positions$$

Since electrophiles attack at positions of high electron density:

$$-OH is an ortho/para director and ring activator$$

Compared to $-NO_2$ which withdraws electrons by resonance and is a deactivating meta director.