Creating Preprocessor Directives for SteamVR and OculusVR

I'm attempting to take a VR project that was written to run with SteamVR to be compatible with OculusVR as well. From what I've read I need to use #define directives, but I'm lost on how exactly to go about doing this. How do I create a #define OCULUS, for example, that will only be defined when I'm using the OculusVR sdk? Or is there a way to create a directive that is defined in an exclusive scene??

 

I've solved this.
I added a define directive for Oculus with the following:
if(XRSettings.loadedDeviceName == "Oculus")
PlayerSettings.SetScriptingDefineSymbolsForGroup(BuildTargetGroup.Standalone, "OCULUS");