How do I lighten oak cabinets?

If the cabinets were sealed, then you would need to rub them down and maybe refinish with a clear wax or maybe a bleached oak wax......Good luck! Enjoy the journey, Do the doors first and that might be just enough to change the look!

Paint. Maybe white wash them.

We wiped Gray General Finishes Gel stain over ours with tube socks...lots of how to videos out there

Start with deep cleaning using a wood safe degreaser. You might need to scrape with a plastic scraper to remove any build up and residue.

If that doesn't lighten enough, scrub with steel wool and then denatured alcohol OR TSP.

You can then either stain, paint, or seal.

Sand down to raw wood, stain, and seal. Lightly sand, prime with Kilz primer, then paint.