[Oberon] excels at really long endless missions, particularly ones where you're staying in one place. Mostly Survivals, Defenses, and Interceptions. You can use him pretty much anywhere though. Maybe not Spys. Please don't bring him on Spys.

Abilities

Smite (With [Smite Infusion]): A minute's worth of 177% extra radiation damage. Maybe not the best damage type, but in most cases, damage is damage, and this is a lot of it, and doesn't need recast too often. This is probably the ability with the worst range and allies often won't get it. I don't think I've ever NOT held-cast it, but you can tap-cast it if you really want to. It's a good Augment, but if you don't like it, this is the ability I would subsume over instead of Reckoning.

Hallowed Ground: It's pretty good any time you're stationary or defending an objective. The default range on it is decent. 2 casts of it facing away from each other will give you 360 degrees of coverage. Mostly use it for the armor when combo'd with Renewal and radiation procs to make enemies fight each other instead of you and your friends, but the status immunity is extremely good against Eidolon magnetic procs and with Hunter Adrenaline, you'll actually get back loads of energy instead of losing it. Cast every half-minute. Or just once to get the armor and go about your business.

Renewal (With [Phoenix Renewal]): You have a massive pool of energy and decent duration, this ideally never turns off. Things to avoid are 1) nullifier bubbles, and 2) falling into pits. Recast if someone loses their buff, you can afford it. This build might actually have too much energy. I used to run [Streamline] instead of [Smite Infusion] but didn't really notice any difference removing it, there's plenty of energy to upkeep all your buffs (and [Smite Infusion] is nice). The armor this grants when used with Hallowed Ground makes your friends less squishy. Iunno if the armor applies to objectives, but the heal does, and it's pretty good at undoing chip damage if enemies are dying. The best part is the extra life aspect for sure, doesn't come up a ton, but when it does, you'll be happy to have it. When you do long arbitrations to the point where it doesn't matter how much health you have and things can two-shot you, this helps a lot. Mostly for squishy friends though, it makes them happy.

Dispensary: Honestly, Reckoning is expensive as hell, the range on it is pretty shite, and the requirement for armor stripping is slightly tedious. This has you covered for health orbs, AND energy orbs and ammo. I tried out Roar and Warcry, and while decent, this one has made me the happiest. If you still want armor strip, you're probably better off getting [Ember]'s Fire Blast or [Ash]'s Shuriken with [Seeking Shuriken] Augment. I just like to lean into the support and people love energy, and this easily procs Arcane Energize for you and everyone else that has it. I do not recommend [Grendel]'s reworked Nourish as a replacement, I find it clunky and winmoar. This gets the energy orbs (and other drops!) in your hand right away and Arcane Energize is already plenty. You and your allies won't be going through energy fast enough to need it.

Mods

[Energy Siphon]: You actually don't benefit from this much, because you can't passively gain energy while channeling an ability. But like I said, people love energy. I suppose if you do somehow run out of energy from a nasty magnetic proc or something, this will eventually get you high enough to cast Dispensary and fill you back up.

[Primed Sure Footed]: It's just a really good quality of life mod. If you don't have it or don't want to use it, use Power Drift for more ability strength and a lesser anti-knockdown effect. Or regular [Sure Footed] if you mainly want the quality of life.

[Hunter Adrenaline]: You know how I said you don't benefit from passive energy gain while channeling? This actually does let you do that if you're getting hit a bunch. Again, you probably have too much energy, but people love energy and you're probably no different. This lets you keep Renewal going, and Renewal helps you get your health back. It's sort of a self-sustaining cycle. I guess it also has a super minor set bonus that probably won't ever come up, but it's there. You can use [Rage] as a slightly cheaper slightly weaker alternative.

[Primed Continuity]: Makes you have to cast less for buff upkeep and use less energy per second while channeling. You don't really need efficiency or range much so, this covers your duration needs. Alternatively, you can use regular [Continuity].

[Primed Flow]: All that energy you keep making has to go somewhere. This just gives you a very generous buffer for it. I greatly enjoy all the freedom having basically infinite energy gives me. Alternatively, you can use regular [Flow].

[Smite Infusion]: This is what makes your 1 useful, big damage boost for you and your friends. Not the best status proc technically, but isn't death the best crown control of all? This makes you make things dead better.

[Umbral Vitality], [Umbral Intensify], and [Umbral Fiber]: Help make you immortal and the set bonus gives you an extremely generous chunk of ability strength (it's basically the only source of it you need, as far as mods go. For more strength, see Archon Shards below). Added bonus of Tau Resistance, whatever the hell that is. Not many frames benefit a ton from all 3, but [Oberon] can definitely make good use of em. TECHNICALLY [Adaptation] is better for effective health points than [Umbral Fiber] but I like the set bonus because it makes my health exactly 1337 and you probably won't die either way, so why not have some fun and get some strength out of it? Seriously, this dude is tanky. Alternatively, you can use the non-Umbral versions of these mods.

[Phoenix Renewal]: This is probably the main consumer of your energy, though not [Oberon]'s only payoff, it's a nice safety net and I find it very enjoyable whenever I see someone with the red flame debuff symbol knowing I saved them for certain demise :)

Arcanes

Arcane Energize: Energy Orbs are the main way you upkeep your energy for casting. Dispensary makes em, you pick em up, and this gives you massive boosts when enemies aren't whacking you hard enough to trigger [Hunter Adrenaline] (which is probably a good thing). The AoE it triggers restores a ton to allies and keeps them spamming... er... whatever the hell they're doing. At least it makes them happy.

Arcane [Guardian]: This is up pretty much all the time, it's just a huge chunk of armor. Armor has diminishing returns, but [Oberon]'s base armor is only 225 (42.9% DR), which is decent, but not exceptional. Not enough to never die. [Umbral Fiber] boosts it up pretty significantly to 658 (68.7% DR), but this gives you another 900 and puts you at 1558 (83.9%), which is 15% damage reduction on top of that. Probably worth it. Because of the diminishing returns, it's not as good to run this and Fiber together instead of one or the other and Adaptation, but if min-maxing isn't the be-all-end-all to you, this definitely gets the job done and then some anyway.

Archon Shards

Crimson is probably best. Toss em all into ability strength, up to 75% more with Tau shards, and get better heals, more armor, bigger [Smite Infusion] damage, and more common radiation procs on grass. If you're using Dispensary, you'll get more drops too. Strength is the main thing I wished I could fit more of into the build and it looks like my prayers have been answered with the advent of Archons. You can use Duration instead, if you prefer to be casting weaker Smite Infusions/Hallowed Grounds/Dispensaries less often, and to get a slower energy drain on Renewal. You may also find Amber Archon Shards put into Casting Speed make his Hallowed Ground feel cleaner to cast, and, if you opt to use Reckoning, make it much more bearable to use. Azure Archon Shards don't have a lot to offer that [Oberon] can't get elsewhere and doesn't need much more of, but I suppose you could opt to put them into Armor and change out Arcane [Guardian] for something else you want. Violet and Emerald Archon Shards have nothing to offer to his basekit, though if you really like to melee, Violet has some large boosts there. There might be some sort of funny build that takes advantage of Topaz Archon Shards to make Hallowed Ground + Reckoning do something kind of impressive, but that's not what I'm here for I'm afraid. Feel free to give it a try if you're into that sort of thing, though.

Notable Mentions

If you require still more energy, [Synoid Gammacor] is a decent secondary for tearing through shields with magnetic procs and has a lovely little passive where you explode with magnetic damage and get some energy from time to time. Your nearby allies get some too. Just more supporty goodness if you're really trying to compete with Trinity for best energy-giver.

[Djinn] has a mod called [Reawaken] that makes it come back when it dies, which is nice for really long missions where sentinels don't last long. If you really want to go all in, you can put a mod called [Sacrifice] on it, so if, somehow, you DO die, it will kill itself to resurrect you, and then it will come back shortly after. It's a bit absurd layers of defense, but if you're committed to the bit: Always live, never die, Tenno. Since companions update, you can actually use any sentinel for this, but I still like Djinn for the higher uptime. Also, Oberon's passive got some love and sentinels actually benefit from the health/armor/shield-link now, making them super tanky since [Oberon] himself is tanky :)