Could not load file or assembly ‘Oracle.DataAccess’ or one of its dependencies. An attempt was made to load a program with an incorrect format

I have installed a Web app on IIS 7.0 Windows Server 2008 R2 64bit.
I am referring an oracle.DataAccess.dll;
When I try to access the application I get the following message:
“Could not load file or assembly ‘Oracle.DataAccess’ or one of its dependencies. An attempt was made to load a program with an incorrect format.”
Can anybody help me, please?