I installed the odp.net 32 bit installation for Visual Studio 2012. I set a reference to the Oracle.DataAccess.dll and my connection to Oracle seems to be working.
When I build the project (.net 4) I get the following error. The project is set to build AnyCPU (my workstation is 64 bit and the server where we will ultimately deploy to is 32bit)
'There was a mismatch between the processor architecture of the project being built "MSIL" and the processor architecture of the reference Oracle.DataAcess, Version 4.112.3.0, Culture=neutral, PublicKeyToken=89b483f429c47342, processorArchitecture=x86, "x86". This mismatch may cause runtime failures. Please consider changing the targeted processor architecture of your project through the Configuration Manager so as to align the processor architecture between your project and the references, or take a dependency on reference with a processor architecture that matches the targeted processor architecture of your project'
This is only a vs.net warning however is there any way to get rid of this?
Like you said, it's just a warning. Because ODP.net is not "AnyCPU" the warning indicates that you have a dependency that is not going to adapt to the host operating system as your own application is. As long as your odp.net install matches the os in terms of bits, you'll be fine. But the compiler is unable to make that determination and is trying to give you a heads up.
I did find a connect article on this which includes a possible change (i'm assuming to the proj file) to disable the error:
<PropertyGroup>
<ResolveAssemblyWarnOrErrorOnTargetArchitectureMismatch>None</ResolveAssemblyWarnOrErrorOnTargetArchitectureMismatch>
</PropertyGroup>
In any case, your "AnyCPU" application will run fine on your server as long as the 32 bit odp.net you install on the server is the same version as the 64 bit odp.net you referenced (or publisher policies are properly installed to redirect to a later version). To eliminate any confusion i generally set "Copy Local" for the reference to "false." In otherwords, I compile against a specific version of the dll but let it run against what it resolves from the GAC (which includes publisher policies that most of the odp.net installations include).
You can also install the 32 bit odp.net on your dev machine (ideally the same version again) in order to run/debug 32 bit applications or to use the integrated tooling that comes "with Oracle Developer Tools for Visual Studio" inside of Visual Studio.
All that said, there's more than meets the eye here. If you're application is in fact running (which is implied with "it's only a warning"), as 64 bit, than it is NOT using your 32 bit installation. I would guess your machine already has the 64 bit version installed (multiple oracle homes).