"How to change lightbulbs. I have a 15" x 50" oak wood frame fixture. There appears to be no way to remove it! "

You don't need to remove the fixture to change a flourescent tube, unless this is a very unusual light. The opaque glass or plastic panel should pop off or drop down depending on how it is secured.

What Roadbike said. The diffuser lifts or swings off. It has to because the bulbs don't last forever.

If you can't get it to budge, look for nearly-hidden metal tabs that you have to pivot out in order to loosen the cover.

Check the short ends for screws also.

many 48 inch fixtures us a screw on each end in the middle to hold the cover to the metal reflector.