Luvadea,
Welcome aboard. If you are looking to weather your ships with Artist Oils, then I agree with GMorrison, take a look at DavidK's threads of his Golden Hind, Santa Maria and his current Soleil Royal as well as mine of the Hanse Kogge, Thomas and Nina. It can answer pretty much most of your questions.
When using Artist's Oils you need to have your base coat an acrylic paint. Otherwise it will attack an enamel or laquer base coat and make a mess of the whole thing. That being said, I am using enamels for the base coat on my PT Boat kit and plan on using Future Floor polish, (Edit) Which is now called Pledge Floor Care Multi-Surface Finish. I think I'll still call it Future.
Now the issue with Future is that the more layers of paint and finish you put on a plastic wood grain, the more you will lose the grain effect, so I would stick with acrylics. I have also used Tamiya rattle cans for base colors and have had no problems with them.
With the Wasa, I would suggest using a Dark Yellow as your base coat on the hull and then overlay that, after the base coat is dry, with Burnt Umber. If you want a little bit of red tone, then add Burnt Sienna. For your decks I would use a base coat of Gray and then a Yellow Ochre with a touch of Burnt Umber. Leave it on for about 20 to 30 minutes and then wipe it off with an old t-shirt.
Now Dave uses his oils as a wash, cutting the paint with turpinoid. I use oils right out of the tube. So, as others have recommended, test on a styrene sheet to get the look you want before you commit it to the model. Don't be afraid to ask for help.
Hope this helps,
Steve