Oppo took the wraps off a foldable smartphone that’s more than $500 cheaper than Samsung Electronics Co.’s Galaxy Fold, launching its maiden attempt to stake out a spot in the nascent category. The Oppo Find N sports a 7.1-inch inner display co-developed with its Korean rival and an exterior 5.5-inch screen. It will start at 7,999 yuan ($1,250) in China from Dec. 23, undercutting the $1,799 Galaxy Z Fold 3. Like the latest iPhones, the inner display of the Find N has an adaptable refresh rate, speeding up when necessary and slowing with static images to preserve battery. Oppo says...