Undertow is a versatile ability and offers many different utilities to Olaf:
It is a very useful ability for chasing because of its own cooldown reduction (if you can pick it up while chasing). Throwing your axe where you are likely to chase your opponent lets you refresh its cooldown much quicker and increases its slow uptime.
It is an effective harassment ability. While many skillshots are consumed upon hitting the first target, Undertow will travel straight through all enemies it hits. At early levels, however, recklessly spamming it will quickly deplete your mana.
It will remove the fog of war in a large area around where it is thrown for a very brief duration, making it useful for checking bushes.
Although Olaf's sustained damage output isn't very powerful during early game, his burst is surprisingly strong as his Reckless SwingReckless Swing deals a significant amount of true damage which cannot be mitigated by any means. With a few ranks invested in the ability it can devastate the health of most squishier champions during mid game.
If you have not invested in significant HP as you level it, it can also fairly quickly devastate the health of Olaf himself if you are not careful with its usage. By this token, it can often be unwise to spam this during a 1v1, as he can potentially wind up helping to kill himself.
Ragnarok can be used either as an initiating ability, as a defense ability in battle or as an escape ability because of the immunity to crowd control effects.
Olaf's strongest role during the lategame is to function as a bulky assassin. One of his most effective strategies during lategame teamfights is to activate Ragnarok, single out the opposing carry or AP mage and quickly slay them before the opposing team can kill him. Because of the complete CC immunity Ragnarok grants him, he becomes extremely difficult for the opposing team to peel from the carry.
Throw Undertow just in front of you to deal damage and immediately pick it up. This is an extremely fast way to jungle after the Olaf changes in V18.104.22.168 while providing a very good initiator when ganking. Reckless SwingReckless Swing does damage to yourself which can make ganking difficult when leaving the jungle with lower health.
Before engaging Olaf, keep in mind that he becomes very strong when he is at low health due to his Berserker RageBerserker Rage. Properly judge if you can burst him down with your abilities, or he may just lifesteal it back and kill you.
Undertow is a slow projectile, meaning that you can dodge the spell easily.
You can deny Olaf from spamming Undertow by preventing him from picking up the axe that the spell left in the path.
In a similar vein, making significant course changes while Olaf is chasing you will prevent him from picking up his axe and continuing to close the distance on you.
Save your CC while Olaf is using Ragnarok. It removes and negates any CC cast on him while it is active.
奥拉夫的Ragnarok also gives him bonus armor and magic resistance. It might be wiser to wait until the buff duration is over before committing to attack him.
As Olaf is immune to any forms of CC placed upon him, the most effective forms of keeping him away are self-displacement abilities such as Arcane Shift and player-created terrain such as Cataclysm. You should try to save these abilities for when he uses his ultimate to make him waste the duration.