One Database connection for all windows forms

Your question make think that you are writing data access code right under the button click or something like that.

Please don't ever to that.

Write separate class for data access, create instances of that class and consume methods.

If it is not clear, please ask.

Just you have to mention connection string in your app.config file that connection string will be universal for all window forms make sure you have oracle client install on your machine in order to setup connectivity with your DB.

For more refer this URL

http://www.c-sharpcorner.com/UploadFile/deepak.sharma00/connecting-to-oracle-database-from-windows-forms-application/