Where could this oil leak originate on this Mazda 6?

That looks to be where the transmission bellhousing meets the engine block. If that's the spot, the leak is most likely the rear main seal. This is where the engine crankshaft comes out of the engine and the place where the flexplate (assuming an automatic transmission) connects to it. There is a seal which circles the crankshaft. A leak where you're showing it is usually caused from this seal leaking.

This is not an easy fix, because one has to remove the engine/tranny from the car (most front wheel drive cars requires this), then split the tranny and engine, remove the parts, replace the seal, then put it all together again. A lot of work for a $20 seal.