The Wise Oak is a unique Bismuth Flask. This flask provides a defensive buff to whichever resistances share the lowest uncapped value, and a penetration buff to whichever resistances share the highest uncapped value. If any or all of your resistances match, they will share benefits.
How to use wise oak effective? As a molten strike user with 40% cold dmg from nodes and 60% fire from molten strike, if you can’t get the right gear to balance all 3 resists should you just make sure your fire resist is the highest for optimal dmg?
Yes having the exact same res for the three resistances is the ideal situation and it is difficult to do but it is not mandatory to make use of the flask. For example if you’re mostly lightning damage having your lightning res the highest will give you the penetration you want and that’s easy to achieve through some gear or a topaz flask.
Wise Oak is probably the best flask in the game for ele attack builds, so having perfectly balanced res is something you should slowly work towards. If you want maximum benefit you have to balance all 3 resists exactly. Easier if you have a lot of master crafted resistances because then you can change or reroll them until you get it.
Most people probably just try to get highest res on their main elemental damage type and not worry about it too much. It’s exactly as good as it’s always been, since it hasn’t been changed (except for the values being lowered slightly, of course). You typically do primarily one element of damage – or possibly two – so getting two resistances balanced isn’t that difficult.
And it’s not that hard at all. You also can get all your resists +/-10 ish of each other with 2-3ish crafted resists on gear (basically every single build has this) and use master crafting to get the right resist values
Also keep in mind that often you’ll be spamming flasks together, for example wide oak with dying sun and taste of hate, in which case you would want your buffed resists to be balanced for maximum effect from wise oak, not the unbuffed hideout resists.
If you want an easy all elemental penetration and all elemental damage reduction from your Wise OAk, only use unique thats gives + all elemental resists. If you are new to the game and don’t want to mess up your first build, follow some build guide from u4gm.