One often starts an oil painting with a wash made from diluting oil paint with a solvent such as turpentine or using a lean painting medium. It is often useful to have a wash to block in the basic composition of the painting before applying thicker and more solid paint. If one wants to avoid using solvents you can use the following techniques when beginning a painting.
The simplest method is using the alla prima or all-at-once technique. The impressionists used this method a lot as one can see from their thick application of paint. All you do is use straight oil paint out of the tube and complete the painting in one sitting. Wet-on-wet painting and blending is fundamental to this technique.
Paint straight from the tube
If you don’t need to start with a wash you can just use straight paint from the tube with no medium additives. There is no need to add anything to oil paint as its consistency straight out of the tube is fine to paint with. Just use your brush to spread and thin paint around the canvas. Transparent and semi-transparent pigments are useful when you want to have more of a translucent feel to your paint. Check the back of the tube of paint to check its transparency. If you wish to thin the paint even more you can use a medium such as walnut alkyd medium.
Oil paint adheres to acrylic paint so you can paint with oil paints over acrylic. Never paint the other way around with acrylic over oils as they will not adhere correctly to each other. You can start a painting with a thin acrylic wash and paint as many layers of acrylic as you want and then switch to oil paints for the subsequent layers of the painting. This has the advantage of the quick drying time of acrylics for the under layers when you are refining the composition whilst also keeping the intense luminosity and blending advantage of oils for the subsequent layers.
Water soluble oil paints
Whilst you can use water soluble oil paints for a whole painting to avoid solvents you can also just use them for the underpainting and use traditional oils and mediums for the subsequent layers. You can use water or a water soluble medium to thin the paint to create a wash just like you would with turpentine and then use a medium such as walnut alkyd medium with the traditional oil paints in the next layers.