For example, I colour a leaf green with the oil pastel. Then I want to spread that green colour of the leaf edges, outside. For this, I use my thumb. I put my thumb on the leaf edge on the paper and them push my thumb forward.
This works, but it also make my thumb dirty. It also means that I will have to wash my thumb with soap every time I want to spread a different colour. Also because my thumb is big, I cannot spread colours of small areas effectively.
What are my options?
One way to simply keep your thumb clean is to wrap it in a paper towel or cloth. This doesn't really resolve the problem of thumb size, however -- indeed, it makes your thumb larger.
For finer detail work, a tortillion would be better. This is a rolled-up piece of paper that tapers to a point. This is much like Zizouz212's suggestion of an eraser, but perhaps less expensive -- you can purchase tortillions, but it's also possible to simply roll your own from whatever paper you have on hand (here's one tutorial).
With oil pastels, I've actually found that using an eraser will help smudge it.
If you have a fine eraser, that's decently thin, you should be able to use it to smudge in small areas. When you want to move on to another colour, you may need to eraser the tip off, but it lasts a good while and it's always given me great results.
You should be able to find an eraser with a small tip. The refillable ones generally have small tips:
Alternatively, you could use a pencil tip, even if it might not last as long.
