Set the Instant Pot to Sauté and heat the olive oil until shimmering.
Rinse and pat the chicken breasts dry, then season both sides with the kosher salt and black pepper.
Sear the chicken in the hot oil 3–4 minutes per side, until golden; transfer the chicken to a plate.
Add the diced onion to the pot and cook, stirring occasionally, until it begins to turn translucent, about 3 minutes.
Add the minced garlic, minced jalapeño, dried oregano, and ground cumin; cook and stir for about 1 minute until fragrant.
Stir in the corn, chicken broth, and chopped tomatillos, scraping any browned bits from the bottom; return the seared chicken to the pot.
Secure the lid, set the Instant Pot to Manual/High pressure for 20 minutes, and start.
When the timer ends, carefully perform a quick release of the pressure, remove the lid, and transfer the chicken to a cutting board.
Shred the chicken with two forks, return it to the pot, stir in the lime juice, and mix to combine.
Ladle the posole into bowls and top each serving with chopped cilantro and diced avocado.