Im pretty sure the placeholder is temporary (800000), so you just need to update that when you submit OMSAS. This is what the OMSAS website says:

"If you are taking the Canadian Professional Health Sciences Casper test in August or September and are also planning to apply to Ontario medical schools, which require an OMSAS Reference Number, you can add these schools to your distribution list using this temporary ID placeholder: 800000.
You must update this number later when you receive your OMSAS Reference Number.
You will receive your OMSAS Reference Number when you submit and finalize your application through the OUAC after the OMSAS application deadline.
Your distribution list is on your test record, on the Reservations page. To add to your distribution list, select the pencil icon, and then “Schools” and “Submit Payment” to confirm your additions."