Prepare a medium charcoal or gas grill fire. Put the corn, onion, and pepper on a large rimmed baking sheet and brush with 2 Tbs. of the oil. Season with 1 tsp. salt and 1/2 tsp. pepper. Grill the corn and onion, flipping occasionally, until beginning to brown (the onions should still be a little crunchy), 6 to 10 minutes. Transfer to a cutting board to cool slightly. Grill the pepper until charred on all sides, about 12 minutes. Put the pepper in a bowl, cover, and cool slightly.
Meanwhile, in a small bowl, whisk the remaining 6 Tbs. oil, the chipotle and the adobo sauce, vinegar, 1 tsp. salt, and 1/2 tsp. pepper.
Coarsely chop the onion and put it in a large bowl. Cut the corn from the cobs and add to the bowl. Skin, seed, and coarsely chop the pepper; add to the bowl, along with the beans, cheese, cilantro, and oregano.
Rewhisk the dressing, add it to the corn mixture, and toss well. Season to taste with salt and pepper. (The salad may be made up to 1 day ahead. Refrigerate and return to room temperature, adding the fresh herbs just before serving.)