Olaf

The passive (Berserker Rage) gives attack speed when you're low on HP — the lower you are, the faster you attack. Olaf is dangerous at low health.

Q (Undertow) slows and if you pick up the axe, the cooldown is greatly reduced — walk over the axes for near-continuous Q.

R (Ragnarok) makes you immune to CC — activate it when you charge the enemy carry so you can't be stopped.

E (Reckless Swing) deals true damage — use it in fights to ignore resistances.

Kite him: without his R, Olaf can be slowed and CC'd easily. After his R, wait for it to expire then CC him.

Don't fight Olaf when he's low on HP: his passive makes him more dangerous at low health, not less.

His R has a relatively long cooldown — track it and engage when it's down.

Ranged champions with CC counter him: he has to run toward his target with no dash.