Check:
Rillaboom

Iron Bands (Rillaboom) @ Choice Band
Ability: Grassy Surge
Tera Type: Grass
EVs: 128 HP / 252 Atk / 4 Def / 124 Spe
Adamant Nature
- Grassy Glide
- Wood Hammer
- Knock Off
- U-turn
With Grassy Glide, Rillaboom becomes an effective check to Ogerpon-Wellspring, dodging a 2HKO from any unboosted move and cleanly 2HKOing back with Grassy Glide or OHKOing with Wood Hammer (although the former is preferable so as to not take recoil). Tera Grass can even be used to OHKO Ogerpon after a single round of rocks or spikes chip. Even if they SD on the switch, Rilla will live any hit including a +2 Crit Cudgel, and if they try to Tera Water Cudgel they die to a single glide. If they Knock Off on the switch, Wood Hammer will still OHKO Ogerpon and even Knock into a Tera Water Cudgel Crit will not kill after grassy terrain recovery, although you cannot Glide in this case. Banded U-turn can also be used if they've taken hazard chip beforehand to kill while also generating momentum.
Tl;dr: There is never an outcome where Ogerpon-Wellspring will be leaving the field alive.
This is not a counter as if you come in on hazards as they SD, you have a chance of dying to +2 Power Whip while never killing back with Glide unless you Tera, and Rillaboom lacks the reliable recovery to act as a defensive switch-in over the course of a game. Investing fully in defences would help with this, but the given spread is designed to outrun uninvested base 100s (i.e. max def Zapdos) and losing this speed is not worth it. Max HP Boots Rillaboom would be more of a counter set, but please don't ever try to run this in any tier.
Iron Bands (Rillaboom) @ Choice Band
Ability: Grassy Surge
Tera Type: Grass
EVs: 128 HP / 252 Atk / 4 Def / 124 Spe
Adamant Nature
- Grassy Glide
- Wood Hammer
- Knock Off
- U-turn
With Grassy Glide, Rillaboom becomes an effective check to Ogerpon-Wellspring, dodging a 2HKO from any unboosted move and cleanly 2HKOing back with Grassy Glide or OHKOing with Wood Hammer (although the former is preferable so as to not take recoil). Tera Grass can even be used to OHKO Ogerpon after a single round of rocks or spikes chip. Even if they SD on the switch, Rilla will live any hit including a +2 Crit Cudgel, and if they try to Tera Water Cudgel they die to a single glide. If they Knock Off on the switch, Wood Hammer will still OHKO Ogerpon and even Knock into a Tera Water Cudgel Crit will not kill after grassy terrain recovery, although you cannot Glide in this case. Banded U-turn can also be used if they've taken hazard chip beforehand to kill while also generating momentum.
If they SD/Power Whip/Cudgel on the switch:
252+ Atk Choice Band Rillaboom Grassy Glide vs. 0 HP / 0 Def Ogerpon-Wellspring in Grassy Terrain: 219-258 (72.7 - 85.7%) -- guaranteed 2HKO after Grassy Terrain recovery
252+ Atk Rillaboom Grassy Glide vs. 0 HP / 0 Def Ogerpon-Wellspring in Grassy Terrain: 145-172 (48.1 - 57.1%) -- 46.5% chance to 2HKO after Grassy Terrain recovery
Even if they Knock Off on the turn after you come in, they will always go down to two Glides.
If they Knock on the switch:
252+ Atk Rillaboom Wood Hammer vs. 0 HP / 0 Def Ogerpon-Wellspring in Grassy Terrain: 315-372 (104.6 - 123.5%) -- guaranteed OHKO
If they have taken prior chip from Hazards:
252+ Atk Choice Band Tera Grass Rillaboom Grassy Glide vs. 0 HP / 0 Def Ogerpon-Wellspring in Grassy Terrain: 292-344 (97 - 114.2%) -- 81.3% chance to OHKO
252+ Atk Choice Band Rillaboom U-turn vs. 0 HP / 0 Def Ogerpon-Wellspring: 282-334 (93.6 - 110.9%) -- 62.5% chance to OHKO
These will both guaranteed kill after rocks or 1 spike, even after a turn of Grassy Terrain recovery.
If they Tera Water:
252+ Atk Choice Band Rillaboom Grassy Glide vs. 0 HP / 0 Def Tera Water Ogerpon-Wellspring-Tera in Grassy Terrain: 438-516 (145.5 - 171.4%) -- guaranteed OHKO
252+ Atk Choice Band Rillaboom Grassy Glide vs. 0 HP / 0 Def Ogerpon-Wellspring in Grassy Terrain: 219-258 (72.7 - 85.7%) -- guaranteed 2HKO after Grassy Terrain recovery
252+ Atk Rillaboom Grassy Glide vs. 0 HP / 0 Def Ogerpon-Wellspring in Grassy Terrain: 145-172 (48.1 - 57.1%) -- 46.5% chance to 2HKO after Grassy Terrain recovery
Even if they Knock Off on the turn after you come in, they will always go down to two Glides.
If they Knock on the switch:
252+ Atk Rillaboom Wood Hammer vs. 0 HP / 0 Def Ogerpon-Wellspring in Grassy Terrain: 315-372 (104.6 - 123.5%) -- guaranteed OHKO
If they have taken prior chip from Hazards:
252+ Atk Choice Band Tera Grass Rillaboom Grassy Glide vs. 0 HP / 0 Def Ogerpon-Wellspring in Grassy Terrain: 292-344 (97 - 114.2%) -- 81.3% chance to OHKO
252+ Atk Choice Band Rillaboom U-turn vs. 0 HP / 0 Def Ogerpon-Wellspring: 282-334 (93.6 - 110.9%) -- 62.5% chance to OHKO
These will both guaranteed kill after rocks or 1 spike, even after a turn of Grassy Terrain recovery.
If they Tera Water:
252+ Atk Choice Band Rillaboom Grassy Glide vs. 0 HP / 0 Def Tera Water Ogerpon-Wellspring-Tera in Grassy Terrain: 438-516 (145.5 - 171.4%) -- guaranteed OHKO
Their strongest unboosted attack into you:
252 Atk Wellspring Mask Ogerpon-Wellspring Power Whip vs. 128 HP / 4 Def Rillaboom in Grassy Terrain: 156-185 (41.8 - 49.5%) -- guaranteed 3HKO after Grassy Terrain recovery
If they Knock on the switch:
252 Atk Wellspring Mask Ogerpon-Wellspring Knock Off (97.5 BP) vs. 128 HP / 4 Def Rillaboom: 131-155 (35.1 - 41.5%) -- 100% chance to 3HKO after Grassy Terrain recovery
252 Atk Wellspring Mask Tera Water Ogerpon-Wellspring-Tera Ivy Cudgel vs. 128 HP / 4 Def Rillaboom on a critical hit: 202-238 (54.1 - 63.8%) -- guaranteed 2HKO after Grassy Terrain recovery
This does not kill after Grassy Terrain recovery, even if they Terastallize and land a critical hit with Ivy Cudgel (the highest possible damage they can do), although in that case you would die to Wood Hammer recoil; this is the best-case outcome for them.
If they SD on the switch:
+2 252 Atk Wellspring Mask Ogerpon-Wellspring Power Whip vs. 128 HP / 4 Def Rillaboom in Grassy Terrain: 313-369 (83.9 - 98.9%) -- guaranteed 2HKO after Grassy Terrain recovery
+2 252 Atk Wellspring Mask Ogerpon-Wellspring Ivy Cudgel vs. 128 HP / 4 Def Rillaboom on a critical hit: 301-355 (80.6 - 95.1%) -- guaranteed 2HKO after Grassy Terrain recovery
+2 252 Atk Wellspring Mask Ogerpon-Wellspring Knock Off vs. 128 HP / 4 Def Rillaboom: 175-206 (46.9 - 55.2%) -- 74.2% chance to 2HKO after Grassy Terrain recovery
They cannot kill you before you Glide them twice, and even if they Knock Off you will 2HKO them with banded into itemless Glide.
252 Atk Wellspring Mask Ogerpon-Wellspring Power Whip vs. 128 HP / 4 Def Rillaboom in Grassy Terrain: 156-185 (41.8 - 49.5%) -- guaranteed 3HKO after Grassy Terrain recovery
If they Knock on the switch:
252 Atk Wellspring Mask Ogerpon-Wellspring Knock Off (97.5 BP) vs. 128 HP / 4 Def Rillaboom: 131-155 (35.1 - 41.5%) -- 100% chance to 3HKO after Grassy Terrain recovery
252 Atk Wellspring Mask Tera Water Ogerpon-Wellspring-Tera Ivy Cudgel vs. 128 HP / 4 Def Rillaboom on a critical hit: 202-238 (54.1 - 63.8%) -- guaranteed 2HKO after Grassy Terrain recovery
This does not kill after Grassy Terrain recovery, even if they Terastallize and land a critical hit with Ivy Cudgel (the highest possible damage they can do), although in that case you would die to Wood Hammer recoil; this is the best-case outcome for them.
If they SD on the switch:
+2 252 Atk Wellspring Mask Ogerpon-Wellspring Power Whip vs. 128 HP / 4 Def Rillaboom in Grassy Terrain: 313-369 (83.9 - 98.9%) -- guaranteed 2HKO after Grassy Terrain recovery
+2 252 Atk Wellspring Mask Ogerpon-Wellspring Ivy Cudgel vs. 128 HP / 4 Def Rillaboom on a critical hit: 301-355 (80.6 - 95.1%) -- guaranteed 2HKO after Grassy Terrain recovery
+2 252 Atk Wellspring Mask Ogerpon-Wellspring Knock Off vs. 128 HP / 4 Def Rillaboom: 175-206 (46.9 - 55.2%) -- 74.2% chance to 2HKO after Grassy Terrain recovery
They cannot kill you before you Glide them twice, and even if they Knock Off you will 2HKO them with banded into itemless Glide.
Tl;dr: There is never an outcome where Ogerpon-Wellspring will be leaving the field alive.
This is not a counter as if you come in on hazards as they SD, you have a chance of dying to +2 Power Whip while never killing back with Glide unless you Tera, and Rillaboom lacks the reliable recovery to act as a defensive switch-in over the course of a game. Investing fully in defences would help with this, but the given spread is designed to outrun uninvested base 100s (i.e. max def Zapdos) and losing this speed is not worth it. Max HP Boots Rillaboom would be more of a counter set, but please don't ever try to run this in any tier.













