Samsung, I must tip my hat to you. The company is doing a better job this year with the Pie update than it has in any year prior. After beta testing Android 9 with One UI on its Galaxy S9/S9+ and Note9 for a bit, it rolled out the stable version to the former on Christmas and updated its forecast for the latter to say it would land in January instead of February. True to its word, it didn't take more than a couple of days for this to happen.
A user in Germany is reporting that the Pie 9 stable OTA has showed up for him. He was part of the beta test though (that's why the size is only 95MB), so we don't know if the OTA is showing up yet for those who stayed on Oreo — though if the S9/S9+ situation is any indication, both rollouts should happen at the same time. Regardless of which version you were previously on (Oreo stable or Pie beta), the new firmware should be N960FXXU2CRLT and come with the January patch.
You can start checking for the update manually now if you live in Germany, but odds are the rollout will be slow. It'll probably take a lot of time to spread to other countries too.