How to flip coordinates when drawing in context?

I create a context from an UIImage, and then I draw into it with

CGContextDrawImage(bitmapContext,
                   CGRectMake(0, 0,
                              originalImage.size.width,
                              originalImage.size.height),
                   oImageRef); 

the image appears upside-down due to the flipped coordinate system in quartz. How can I fix that?