What happened to my car?

I drive a 2002 Celica GT. A few weeks ago i noticed a slight flutter as i throttled the gas. I inspected the throttle body, and air intake flap, and could not find any hang-ups. Also plugged a OPD II sensor in and got no codes, misfires, etc. The car has been driving fine, other than a flutter. Even noticed better gas mileage….until today. It died on the freeway with a rumble, the oil level light came on; then the Check Engine light followed as the car shut down. Coasted to the shoulder and stopped, checked the oil level….again. and it was normal. It is due for an oil change…was going to do that tomorrow. plugged OPD back in, still no codes. Any ideas?