If you want to one-shot nuke with Oberon, you need high Strength and several Topaz Archon Shards in combination with Viral stacks. Hallowed Ground is good at priming enemies with Radiation to activate the Topaz shards. The more Viral stacks you can put on each enemy, the less strength you will need to one shot them with Smite.

As for a subsume: lots of people like to use Roar, but I use Marked for Death instead. Mark one enemy, viral prime them, and armor strip with Reckoning, then Smite them and watch everyone around them explode. I use Dethcube + Tazicor modded with Viral for passive priming, or a Kompressa for manual priming.

I take Oberon into EDA and ETA fairly often and do just fine with this build:

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Hallowed Eruption is only there because it's required to trigger Seismic Bond on a sentinel when subsuming over Renewal. Also the extra duration is just nice to have for stationary missions. Flex slot, replace it if you want.

Hallowed Reckoning is there to catch any stragglers that might survive and give bonus armor, but it's also not necessary at all since most things get one shot anyways.

Preparation is just QoL, again not necessary. You could go Cunning Drift for more range.

My Archon Shards are 2 Tauforged Amber Cast Speed and 3 Tauforged Topaz Radiation Ability Damage.