Olaf is strong early game with his axes and true damage. Stay farming and throw the axes on the enemy. at lvl 2 you can throw axes and use true damage. Make sure all the time picking up the axe if you can, it will only be 1-2 second/s cooldown on it, and you can pick up easy kills in the start since axes are slowing alot. using exhaust will really secure the kill.
Capture the altars so often you can, and try to always kill Vilemaw, it is worth 190g and gives you a buff that grants bonus stuff