Allan, for God's sake put a set of steel strings on your OO-18 (I would recommend a light gauge phosphor-bronze such a D'Addario's J-16 set). It was made for steel strings and anything lighter is not only going to sound worse, but you may do serious damage to a wonderful instrument by not loading it correctly. If it's had those nylon strings a long time, you are lucky if it still has a straight neck, and it may take it a while to start sounding good under the correct load, but please don't endanger such an instrument. We all want it.

Chet W.