How to Lighten dark oak coffee table?

You can sand the finish and restain it a lighter color....or you can try using straight ammonia to scrub the table. Sometimes, it will lighten wood finishes depending on how it was applied. Be sure to use a clean sponge to rinse well after using the ammonia.

The only thing I can think of is to sand the wood and stain it a lighter color. If you don't want to sand and stain, you could white-wash or paint it.

Sand it to bare wood and then stain a lighter shade.

Yup- sand and restain. You do not have to have matching woods in your new room. It's called Eclectic. The coffee table may end up being a focal point if it does not match.

You’ll go thru a lot of sandpaper trying to sand the finish and stain out of the wood! Gums up sand paper quickly! Get some paint remover like the Orange remover that works great and don’t smell too bad! I’ve used it many times! Make the job go much faster and easier then just trying to sand it all off!

Have you thought about antiquing . There are lots of beautiful colors now.

There's a great tutorial on Hometalk that shows the sanding/paint-washing makeover process for an old coffee table: https://www.hometalk.com/diy/living-room/furniture/coffee-table-makeover-44693677