I am looking at a May bike and sail trip that is between Sweden and Denmark. It looks very interesting, but I am prone to seasickness and I wondered if anyone has been on that body of water . It is called Oresund. I know it is hard to predict the weather but I wondered if anyone has any experiences to share. Thank you!

Assuming that by "sail" you mean on a sailboat you are better off by asking people that actually sail in the area.
The more details you can provide about the boat the better.

https://www.cruisersforum.com/forums/f2/

It may depend on the currents more than the boat. For instance, in my experience, on a 30' sailboat between LA Yacht Harbor & Catalina Island, the current sailing west to the island is choppy, coming back it isn't at all. I think you'll need to find someone with experience on that particular stretch of water.

Is it the tour offered by Boat Bike Tours? Ask them.

The Baltic is a strange body of water. Tideless, the main issue will be wind. As you say forecasting wind is a waste of time.

Öresund is a fairly shallow strait between Sweden and Denmark. If you are prone to seasickness, go sailing on a calm day. As mentioned above, it's the wind that can make it choppy and it can get pretty nasty when the wind whips up.

Going sailing on a calm day is like taking your pet eagle for a walk.
Better to go on a stormy day, one is too scared to be seasick.
Go with him.