When configuring Omnissa UEM you seem to be able only to select the top OG in a SaaS environment when selecting e-mail based enrollment, this is called auto discovery, see https://docs.omnissa.com/bundle/WorkspaceONE-UEM-Managing-DevicesV2306/page/ConfigureEnrollmentOptions.html when completing the FTU for e-mail based enrollment afterwards you can go in and select the OG again an then drill down in the desired OG. Pretty straightforward.
Hope it helps.