Try tightening all the bolts for the oil pan.
If that doesn't help your oil pump is probably leaking from somewhere, either its gasket that bolts it to the front of the engine, or the oil seal for the lower timing belt pulley. To remove the oil pump you will have to remove the timing belt first, my website has the replacement procedure for getting the timing belt off and back on in the
BELTS section:
http://home.comcast.net/~lj_robins/99suzuki/
Once the belt is off remove the lower crankshaft gear then remove the oil pump, the oil pump assembly is bolted onto the front of the engine and is held on by seven bolts. Whether the gasket is bad or the oil seal is bad while you have it off replace both (you have to replace the gasket anyway).
Kind of up to you but while you have it this far taken apart you could replace the water pump as well.