Oil into the cylinder

Hello and welcome to FixYa!

The cylinder head is located on the center of the engine. Yes it can be a worn out head gasket or piston rings. If the piston rings are worn out the oil is burnt that's why it emits white smoke. You will need to replace the head gasket and piston rings.

Hope this helps and thank you for using FixYa! Have a good one!

your Daewoo has an interference engine, when the timing belt brakes the valves hit the top of the pistons, and get bent. you will need to remove the cylinder head an replace the valves.....sorry.

TIMING BELT
Removal
Disconnect negative battery cable. Disconnect intake air temperature sensor connector from intake air duct. Disconnect the air intake tube from the throttle body. Disconnect the breather tube from the valve cover. Remove resonator from throttle body. Remove air filter housing.
Raise and support the vehicle. Remove right front wheel. Remove right front splash shield from wheelwell. Remove serpentine accessory drive belt.
Remove crankshaft pulley bolts. It is NOT necessary to remove crankshaft sprocket center bolt. Remove crankshaft pulley. Lower vehicle.
Support engine assembly from above with Support Fixture (J-28467-B). Remove engine mount bracket from timing belt end of engine.Remove power steering hose clamp bolt, and position hose away from timing belt cover. Remove front timing belt cover. Using crankshaft center bolt, rotate crankshaft one full turn clockwise (as viewed from timing belt end of engine).
Align mark on crankshaft sprocket with notch at bottom of rear timing belt cover. See Fig. 4 . Align camshaft sprocket timing marks with notches on valve cover. See Fig. 5 .
CAUTION: Engine damage could result if camshaft sprockets are not properly aligned with notches on valve cover.
Loosen the automatic tensioner bolt. Use an Allen wrench to rotate tensioner and relieve tension on timing belt. Remove timing belt.
Fig. 4: Aligning Crankshaft Sprocket Timing Mark
Courtesy of DAEWOO MOTOR AMERICA, INC.

Fig. 5: Aligning Camshaft Sprocket Timing Marks
Courtesy of DAEWOO MOTOR AMERICA, INC.

Installation
Ensure crankshaft and camshaft timing marks are still aligned. Install the timing belt. See Fig. 7 . Turn the timing belt tensioner counterclockwise to tension the timing belt. Turn tensioner until the pointer aligns with the notch on the rear timing belt cover. Tighten automatic tensioner bolt to specification. See TORQUE SPECIFICATIONS .
Using crankshaft center bolt, turn crankshaft clockwise (as viewed from timing belt end of engine) 2 full revolutions. Ensure timing belt tensioner pointer is still aligned with notch.
If timing belt tensioner is correctly aligned, install timing belt cover. See Fig. 3 . Install the engine mount bracket on the timing belt end of engine. Position power steering hose in place, and install clamp bolt. Install crankshaft pulley. Install serpentine accessory drive belt.
Install right front splash shield. Install right front wheel. Install air filter housing and resonator. Connect air intake temperature sensor connector. Connect negative battery cable.
Fig. 7: Identifying Timing Belt Routing
Courtesy of DAEWOO MOTOR AMERICA, INC.

here

http://www.2carpros.com/questions/daewoo-leganza-2000-daewoo-leganza-timing-marks

That would be my first stop in this situation assuming you also changed the oil filter when you changed the oil.

Check the cam and see if it has a grove worn in it if so the seal wont work unless you remove the spring from the seal and find the join untist it to seperate it the cut a couple of mm off the replace it to the seal

If you know for sure you have timing belt set right, do a cylinder compression test, if your belt when bad on you , you could have bent valve, this engine is an interference engine, it will bent valve's do a compression test on cylinder's.in no compression an any cylinder's bent valve's.

There are only two valid reasons for low oil pressure...bad bearings on crankshaft or obstructed oil pickup screen for pump...pump can be damaged by debris or hard carbon from either source but generally will not fail on it's own.
You may not be happy to hear this answer, but that's how it works on every engine, not just yours!!!
Good luck with it

where is oil pressure switch/sensor on 2001 nubira

You may have a serious problem (timing belt tensioner). This tensioner keeps the belt 'taut' so it won't jump time. Daewoo engines are 'interferrence' engines (this means if the valve timing is 'off' (out of syncronization) you can damage/bend the valves in the head -- valve timing (timing belt ensures this). The solution is to take the vehicle into the shop and have it diagnosed to be sure/to know if any damage has occured -- if none, replace the tensioner... it's pretty simple to do, if you have the service manual.

The one in the wife's car slipped due to the front idler pulley falling apart and bent numerous valves both inlet and exhaust. A broken timing belt will give the same result.....An expensive exercise to fix. Tony