Oil is mixing with coolant in my 2015 Holden Barina

This is not a good sign and suggests that you have a fairly major engine problem. Sometimes this can be caused by an automatic transmission’s oil cooler having split and emptying transmission fluid (oil) into the radiator’s coolant. If that’s the case, the transmission will also probably be damaged both mechanically and electronically.

Other causes are a catastrophic failure of the engine crankcase or – not quite as serious – a blown cylinder head gasket. But in either of these cases, the engine will need major repairs. The cost of which may exceed the actual value of the car.

Depending on the severity of the problem, a recycled, second-hand engine from a wrecked Barina of the same model might be a better option.