IT could be a million different things, the best thing to do aside from figuring out what happened to the coolant line (did some one cut it or did it just fail on it's own???) but you should clean the engine with some purple power citrus cleaner (my favorite to use) and then start the engine and look for the leak (but unless your engine was horribly dirty from loosing all 4+ quarts, you should be able to trace the leak based on the oil that's leaked out of it)
From top to bottom it can be
oil cap
valve cover
head gasket
cam seals and cam angle sensor seal
turbo feed line
oil pump
front cover
oil pan
turbo oil drain
not many places for oil to leak from really, compared to coolant anyway

best of luck (and this is another reason i wish DSM's had a low "oil level" light/sensor instead of 2 oil pressure sensorsfor a light and gauge)