Your oil painting's worth is going to be dependant on the quality of the work, the quality of the materials, and where this painting is being shown at. It's worth will also be dependant on how well known you are, and how well your other paintings had been received. An unknown artist is going to get next to nothing for the same painting that a well known artist would get paid megabucks for. One thing that you could do is to have your painting appraised for value. You should be able to find an appraiser in your local phone book.
To find out the value of your oil painting, familiarize yourself with the artist, condition, and rarity of the work. Research online or at your local library. Seek the advice of an auctioneer or a professional art appraiser if it is valuable. You can find more information here: http://www.askart.com/AskART/help/what_is_my_art_worth.aspx