I have used oil paint a lot in my work, You can use it either way. If you like it to be thick to create texture, use it as it is. If you wish to use it thinned down like a water colour effect, use white spirit. I think it's more fun to use it as it is really, I love to see texture in my work personally.
This is the good thing about oil paint, you can choose how thick/thinned you want to use it.
Hope this helps
