Is it a burnt oil smell or an unburnt oil smell. I have a 95' 325i and I had an unburnt oil smell. The first leak was out the bottom of my dipstick where it goes into the oil pan. The second was just vapor coming off of the valve cover. There is a hose that takes it somewhere, it had been eaten by years of motor oil vapor, and had a big hole in it. It was sending it straight in through the vents.
If it's a burnt smell, it could be an oil leak out the valve cover leaking onto your exhaust. If you've never changed your valve cover gasket, the rubber gets really hard and could have started leaking.
Hope I helped, it is probably a bit different on the 320, but just a few things to keep in mind.