Heat olive oil in large skillet over high heat.
Sear all sides of pork roast until browned (1 - 2 minutes per side).
Pour enough BBQ sauce into bottom of slow cooker to just cover the surface.
Add pork and top with BBQ sauce (reserve 1 - 2 tablespoons for topping at the end).
Cook on low for 6 - 7 hours or 3 - 4 hours on high, or until fork tender.
Heat oven to 400º.
Lay tortilla chips on a large cooking sheet, cover with pulled pork.
Top with red onion and cheddar cheese.
Bake for 8 - 10 minutes, until cheese is melted and chips are golden brown.
Drizzle with remaining BBQ sauce and top with green onion.