How can you fix it if you see engine oil in a couple of spark plugs in a 1990 Honda Accord 4-door LX?

Underneath the valve cover there are 4 grommets that seal the spark plug holes. They cost about 3 bucks each, so it's a pretty inexpensive fix, but be sure to get a gasket for the valve cover as well, since that will need replaced after you remove it to replace the spark plug grommets.

along with replacing the spark plug grommets you will want to really goop them up well with a agsket sealer i usually use a high temp blue if you dont use sealer then they will leak again

also could be worn piston rings

Gasket sealer shouldn't be necessary if you are replacing all of the gaskets associated with the valve cover. Clean the surfaces of the engine block and the vavle cover and put it back together.

Torque the cover down evenly. Don't just tighten one bolt and then move on to the next one. Tighten each bolt a little at a time and be sure to stagger them as you would when tightening up the lug nuts on a wheel.