I want to paint water based paint over oilbased paint without sanding?

In my opinion I would not do it. The water base will peel off the oil based paint. You would need to sand it. Another option is to ask your hardware store if they have a primer you can use with your paint.

This is what you need to know about painting over oil-based paint:

https://www.bobvila.com/articles/painting-over-oil-based-paint/

Hi Hester, it's never a good idea to use water based paints over oils. The finish won't be lasting since the oil-based paint forms a barrier that prevents water-based paint from adhering properly. Unfortunately you'll need to do a bit of prep work first, which does involve sanding I'm afraid.

Hester: maybe this will help

https://www.bobvila.com/articles/painting-over-oil-based-paint/

Its a process, but if you want a long lasting job that looks nice, please do the steps. We made the mistake of painting over oil based paint and it took many, many coats. The extra time and expense of more paint would have been saved by taking the time to sand.

Wash down with TSP then Prime with oil based primer. Its made to accept latex. Talk to your local stand alone paint store for the best primer to buy. If your old walls are shiny you really need to lightly sand.

I found these ideas for you Hester, hope it helps you.

https://www.bobvila.com/articles/painting-over-oil-based-paint/

https://www.youtube.com/watch?v=naig5IoOSf8

To make it right you really will need to do some sanding and use a primer before painting with water-based paint

No sanding. Make sure the walls a clean. Prime with Kilz primer then paint your color. Kilz will seal the oil paint and any paint will bond to it. I always prime before I paint anything.

Get a good stain blocking, bonding primer. Prime the walls and then you can paint over with latex. You do not need to sand with this method, but you will probably have to paint 2 coats. Get either Kilz or Zinser 123.

I wouldn't sand. First I would clean the walls. A damp microfiber cloth will work on most rooms. In a kitchen, I would clean with a TSP alternative like Krudkutter. After they were clean and dry, I would seal them with Kilz. Large stores like Home Depot and Lowe's will tint Kilz to a shade similar to what you are going to paint with. This will save you paint and also aggravation! Make sure any repairs, like small holes, are repaired before painting. Good luck and stay safe!