this website (and others) suggest using white bread very gently http://www.ehow.com/how_2174318_clean-oil-painting.html
I have seen an expert on antiques roadshow clean a small corner of an oil painting with a cotton bud moistened with his own (clean) saliva.
If you treasure that painting then a professional clean is probably the best idea though
Oil paintings are usually varnished to protect the paint,if you bought this from a street painter this may not have happened. The varnish is applied after the oil paint has completely dried,and this can take weeks or months depending on the thickness of paint.
If it hasn't been varnished then you are cleaning the actual paint,and as woofgang says, a job for the professional.