This is probably because OSCAR can see the older Sleepyhead data and wants you to convert it over to the OSCAR format.
I have no experience with that but someone will be along very soon who can get you past that hurdle.
Unfortunately no-one stepped up to the plate to explain how you can convert the Sleepyhead data over to the OSCAR format.
I don't want to recommend that you uninstall Sleepyhead and delete all its data because I suspect that is overkill and unnecessary.
In addition, I wouldn't know where the Sleepyhead data is stored in the first place.
You may get better assistance by posting your question in the software-support section
Alternatively, as suggested by DeepBreathing... just keep using Sleepyhead. But that feels like a cop-out to me.
It's your choice whether to go down the software-support route or just keep using Sleepyhead (or maybe both).
Sorry I don't have the exact answer for you.
OSCAR uploads - Organize Charts - Removing Calendar and Pie Charts etc to show the Settings
OSCAR uploads - Attaching Charts to your post
OSCAR help pages
How to obtain your Clinician's manual: Click me and go to "Section Three" on this page
Data Interpretation
Treatment-Emergent Central Apnea
Machine Choices - Good and Bad
Download Polyman (EDF Viewer)
Which ResMed machines support which modes (scroll to final post)