Pictures with oil pastel crayons seem light and transparent, but contrary to the lightness of water colour pictures,oil pastels are never that colourful and often seem to be a bit hazy and dizzy. It takes a lot of practice to be able to wipe the colours into shape. But as the colours have very pure pigments in them, you can do very soft colorations by wiping them into each other. The ground is always visible, so it plays an important part in how the crayons come out.
For my part I don’t do complete pictures only with coloured crayons, but I like them for two different purposes: The oil pastels are ideal for outlining small items in acrylic pictures and then do shadows in acrylic pictures.
They are also nice to use together with water colour pencils, doing bigger spots of colour, like in the picture below.
All in all, water colour pencils have a wider range of possibilities.