Olaf is a Blue Moon CO who shines in snow, suffers in rain, and is otherwise average.
Playing as Olaf
Being an all-around average CO, Olaf will play very similarly to Andy, with differences only taking hold as CO Powers begin to take effect. As such, Olaf will typically fare best playing to his opponent's weaknesses, if any exist. Olaf will generally want to play it safe until he can find a good opportunity to use Winter Fury. Due to his strength in Snow, Olaf may slightly favor Indirect Combat, attacking from afar while the Snow restricts enemy movement, making it difficult for enemies to close the gap or retreat effectively. Olaf is just as comfortable with Direct Combat however, so he should still be fielding a balanced force of units.
Blizzard is a fairly basic CO Power, slowing other players down for a day, friend or foe (with the exception of other Olaf players). This can produce a variety of useful results, such as stalling an enemy's advance, or allowing Olaf to be more lenient with unit protection, as certain avenues of attack will become difficult or impossible while it is Snowing. Take note that Land units can still easily traverse Roads and Properties even during Snow, so those routes should be taken into account when positioning. Despite Winter Fury's strength, Blizzard should not be shunned if it can prove valuable in protecting your own units or gaining an advantage.
Winter Fury does has all the same effects as Blizzard, but also damages all enemy units for 2 HP upon activation. Because of this, Winter Fury is often regarded as one of the stronger Super CO Powers. It can easily give Olaf a massive advantage when used well, through letting him do huge amounts of damage to his enemy while making it very difficult to reinforce or counterattack at the same time.
Playing Against Olaf
Generally speaking, you want to take advantage of Olaf's weak early game as soon as possible, before he has time to prepare Winter Fury. Keep your units closer than usual in the process however, as units that are too far separated may not be able to reinforce one another should Olaf decide to play Blizzard instead. Air units in particular have trouble moving in Snow, so keep them from flying too far ahead of any support. Forcing Olaf to use Blizzard may prove advantageous though, for if he is unable to do much damage with Blizzard and it will set him back in progressing towards Winter Fury.
Should Olaf have Winter Fury available, however, the best thing to do is to protect your units as much as possible. If you cannot do much damage to Olaf's forces, the best course of action is to fall back into a defensive position. Taking 2 HP damage stings, but is far better than possibly losing a large chunk of your army in the process. Olaf may be very aggressive following Winter Fury, so he may leave some openings in his defense if he is careless. Alternatively, if possible, it may be preferable to do as much damage possible to Olaf's forces before he has a chance to use Winter Fury, if the attacks are relatively safe. Either way, the largest goal against Olaf is to survive Winter Fury, after which he returns to his average day-to-day stats, taking considerable time before being able to use Winter Fury again.