Walls have been painted with oil base. Can you paint over them with latex without primer coat?
I don't recommend doing this. Even the best latex/acrylic paints have a hard time sticking to hard oil base paint. You only have one chance to do anything right and painting is no different.
The risk is that some paint will stick well and some won't. The result is a wall that is easily damaged and paint that will chip off or possible peel sometime in the future. If you use a good oil based primer first the paint will adhere properly.
Review the following articles for more info.
1) <a href="http://questions.house-painting-info.com/latex-over-oil-paint.html">Latex Over Oil Paint</a>
2) <a href="http://questions.house-painting-info.com/repainting-over-old-alkyd-paint-with-water-based-primer.html">Repainting Over Old Alkyd Paint With Water Based Primer</a>
3) <a href="http://questions.house-painting-info.com/painting_question-wall-latex_over_oil_base_paint.html">How do I re-paint with latex over oil base paint on my walls?</a>