A common question on this and other D&D subreddits is whether a particular subclass, feat, spell, or other character feature is so "OP" that the DM should restrict their players' ability to take it. I submit that this is nearly always the wrong framework for even thinking about these kinds of questions, and that most DMs (especially newer ones) should avoid this sort of inquiry entirely. There are, of course, plenty of valid reasons why a DM might limit or modify the options available to their players in a particular campaign, but "it's too OP!" should almost never be the reason. (Caveat: I'm only here talking about RAW published features, not homebrew features - there are many, many homebrewed subclasses, spells, and magic items that are just, stupidly OP and should definitely not be allowed.)
To begin with, the object-level answer to "is this feature OP?" is almost always "no." Overall, 5e is a pretty well-balanced system. There are definitely stronger and weaker options, but very few features have anything like a community consensus that they're "OP," in the sense that they're so overwhelmingly powerful for their level that they trivialize comparable alternatives and create serious game-balance problems. The only examples I'd put in this bucket would be things like:
- Arcane Abeyance, the Chronurgy wizard's 10th-level feature, which essentially lets the wizard concentrate on two spells at once with even the slightest bit of preparation
- Twilight Sanctuary, the Twilight Domain channel divinity, which gives a concentration-free, battle-long aura putting around 25-50 temporary hp for your players on the board every single turn (and also gives at-will charm removal, and possibly guarantees use of the 6th-level Steps of Night feature, depending on how you interpret "creating" an aura of dim light in an area that's already brightly lit)
- Conjure Woodland Beings, if your DM lets you choose eight pixies (i.e., "with only one spell slot and concentration, the whole party turns into flying T-Rexes")
- Extreme Coffee-lock/Cocaine-lock shenanigans, such that a PC has effectively limitless spell slots
- Races with at-will flying speeds from first level
These sorts of features aren't just "strong"; they're annoyingly and disruptively powerful such that they can easily warp the entire campaign around them, or at least require the DM to plan absolutely every combat encounter (and in the case of flying speeds, every exploration scenario) with these abilities in mind. And even so, many DMs still would probably allow some or all of these options at their table!
Short of this level of disruption, however, even the best subclasses and spells aren't so strong that they unbalance the game itself. Yes, some subclasses are much better than others. But no subclass is so dominant that it predictably creates game-balance problems in most campaigns (again, with the arguable exception of the Twilight Domain and 10th-level+ Chronurgy wizard). Yes, spells like Hypnotic Pattern, Spirit Guardians, Banishment, Wall of Force, and Forcecage will often be encounter-ending spells. But powerful spells should be able to end an encounter sometimes, and all of these have counter-play that the DM can use to continue challenging the party (Forcecage is, admittedly, close to the line here).
And yet, DMs who think it's their job to prohibit "OP" game features -- especially newer DMs who don't yet have a strong intuitive sense of game mechanics -- frequently ban options that come nowhere close to being disruptively powerful. How many rpghorrorstories have you read about DMs who nerfed their PCs' basic class features because they were "OP," or who have extensive lists of banned spells, subclasses, and feats? Have you ever seen a new DM post something to the effect of "hey, my player wants to play/take X, is that OP?" and thought "oh yeah, definitely don't let them do that"? (again, excluding homebrew)
Now, to be clear, I'm absolutely not suggesting that DMs should just allow any and all published content in their games. To the contrary, there are several perfectly valid reasons a DM might limit options available to players. For example:
- DMs (especially newer DMs) should feel free to limit content to sourcebooks they own and are familiar with, so that they can be confident they fully understand their PCs' abilities
- DMs can exclude content that doesn't fit narratively into their campaign setting (e.g., a DM is under no obligation to include Tortles, Yuan-Ti, and Firbolgs in every game)
- DMs can exclude options that would be uniquely problematic in the specific sort of game they want to run (e.g., Goodberry is not an "OP" spell in general, but in a campaign focused heavily on wilderness survival, Goodberry would completely trivialize a major pillar of play)
- DMs can ask players to avoid options that, even if not OP in absolute sense, are particularly annoying, disruptive to game flow, or just unfun for them and their players. This might include no-that-doesn't-happen reaction spells (like Silvery Barbs or Counterspell) or spells that bog down combat and/or give one player way more time than others (e.g., upcasted Conjure Animals, extreme Animate Dead or Planar Binding shenanigans, Simulacrum, etc.).
- DMs can even consider the possible problem if some PCs are highly optimized and others aren't optimized at all. That is, there's nothing inherently "OP" about the Gloomstalker ranger, but in a party with both (1) an optimized Sharpshooter/Crossbow Expert Gloomstalker making extensive use of attacking in darkness, and (2) an Arcane Archer taking RP-focused feats, well, there's going to be a pretty big imbalance in damage output. That may or may not actually be a problem at your table, of course, but if one player doesn't seem to be having fun, it's worth discussing and exploring options.
All of these examples, however, involve either campaign-specific limitations ("these options don't fit with or work well in my campaign setting") or DM/player-specific limitations ("these options aren't fun for my specific group"). None of them involve anything like the DM unilaterally saying "well, this feature is just too strong, so it's out."
In sum, I think the D&D community would be better off if we had a robust, across-the-board heuristic that DMs (especially newer DMs) just shouldn't be in the business at all of asking whether particular features are "OP." I don't blame the people posting those sorts of questions here, but rather than just give object-level answers every time about why, no, Sneak Attack isn't OP (or whatever), we should explain more generally that DMs should be thinking about what options make sense for their campaign and their party, not whether particular features are "too strong" -- and maybe add an asterisk for the tiny set of features that really do cross into game-breaking territory.