It depends upon two things: how intensive the database operation is and how
often it is requested.
Oracle runs as unmanaged code. WinForms is managed code. The transition from
managed to unmanaged and back is a significant time sink. I ran a time test
last year on this and got unbelievably bad numbers - something in the order
of a 100 milliseconds to make a call from a WinForm to a DCOM server
function on a remote Windows 2003 server that did nothing except return.
Having said that, assuming that each of your users is using a dedicated copy
of your WinForms application and your Oracle database is being served by a
reasonable piece of iron and the database operations are not out of this
world intensive, then you shouldn't have any trouble for normal operations
of several hundred concurrent users.