In a 2-quart saucepan over medium heat, combine the beans, olive oil, onion and garlic powders, chipotle, oregano, and 1/4 cup water. Cook, mashing with a potato masher, until the beans are hot, well mashed, and thickened to a spreadable consistency, about 3 minutes. Season to taste with salt.
Stack the tortillas 3 at a time and prick through the stack in several places with a fork to prevent the tortillas from puffing up as they fry. Add enough oil to an 8- to 9 inch skillet to measure 1/2 inch deep (about 1 cup) and heat over medium-high heat until shimmering hot. Add a tortilla and cook, flipping 3 to 4 times with tongs and gently holding the tortilla under the oil until it’s crisped and golden-brown, 2 to 3 minutes total. Transfer to a paper-towel-lined plate to drain. Repeat with the remaining tortillas, adding more oil as necessary.
Serve the tostadas slightly warm or at room temperature, each spread with some of the beans and drizzled with a little crema or a dollop of sour cream, if you like.