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It’s a hard concept that you’ll have to figure out by yourself in time. Because not every build has same needs. There are too many variables to say you need this much of that etc.
But let me clarify OA/DA issue for you.
OA basically determines your hit chance and crit chance. It is needed to hit cirt or to procs that are based on chance on crit hit. If your character has good crit damage to utilize or important procs that need crit hit to proc; you’ll definitely need 3k OA. But there is also debuffs like DA reduction. So if your character has 250 DA reduction for example, you can live with 2750 aswell. Though extra OA never hurts, if it’s not crucial to your build it’s not worth to sacrifice something in order to increase OA.
Note that with too low OA you might miss your attacks againts enemies have OA shred or high DA in end-game. So 2700+ is essential.
DA is the same thing for enemies perspective. It determines enemies hit&crit chance to you. Most enemies have not high crit damage to begin with up to a point and their hit chance never will be less than %60 so stacking 4k DA won’t make you immortal. On the other hand in high SR their OA and crit damage will increase so you also won’t want to get critted by some high hitter.
To understand how much DA you need, you can check your other defensive mechanisms.
My rules are these;
Does build have high armor(over 3k) and enough phys resist(over %30)?
Does build have overcapped all resistances atleast %25-30(preferable value is 40 since a few boss have %40 RR and you may encounter them as combinations/pairs.)
Does build have atleast 12-13k health?
If you can check these stats you don’t need 3k DA. You can endure some extra 1k damage. Ofc all these speculations based on you already have good sustain with lifesteal or health regen. Even if you left 1k health after one single hit, if you can regain the health quickly you won’t die.
So the mat is basically this; if your build endure more hits/crits you don’t need to stack DA. There are builds that can survive in SR 90 with 2500 DA. But to be safe I recommend you atleast 2750 aswell. In SR 75-76 nemesis will start to crit you with 2850 DA; since some of them have high DA shred. 3k will ensure that they won’t crit even when they debuffed you. But as you proceed in SR, their OA and Crit damage will increase so they will have more crit chance and more crit damage againts you.
But, there is also two other important defensive mechanism;
Damage Absorption; Flat damage absorption is the best, so skills like Inquisitor Seal, Ascension, Blast Shield etc can make you very tanky on their duration. If you have reach to those skills, you may ignore some defenses to get more damage. There is lso % damage like Maiven’s Sphere of Protection or Obsession skills.
Damage Reduction; This is also very important if you’re aiming for end-game. Even if you have good defenses, some monsters hits very hard and without reducing their damage you may not tank them. Constellation like Light of Empyrion is very good for DR, it’s AoE %24 DR, it basically means the monster get debuffed with this will deal %24 less damage.
So as I said all of these are connected to each other and can not be approached independently. If your build has good defenses, you may ignore DA.
You can also check monster scaling and their threats on here https://www.grimtools.com/monsterdb-sr/
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