Oil in coolant resvoir - 2007 Chevrolet Aveo Sedan

Not good! The coolant and lubricant in the engine should never come in contact with each other.

Check the oil dipstick - if it is white / foamy white - is indication of coolant entering the crankcase and mixing with the oil. This can happen if the engine block has cracked or head gasket is torn / damaged.

After the engine has been running / warmed up completely (you get full heat from the heater or thermostat indicates in the normal range), look at the tailpipe. There should be no visible "smoke." If you see white "smoke" it is probably coolant in the form of steam / water vapor and confirms the crack / head gasket issue. Low coolant / oil levels can also indicate this.

Neither problem is an inexpensive repair - but the a cracked block is usually the "point of no return" that few will fix unless the car is in great shape, low miles, etc.