Maybe I'm missing what you are asking, but it looks like you answered your own question when you said "The 990 can only output the OSD in analog" which is correct. You will need to hook up one of the analog outputs of the 990 to the Sony to see the OSD. The DVI jacks on the 990 are just a switch nothing more.

In fact if the Oppo is your only digital output device I'd recommend you not connect it through the 990.