Since it is a flash chip we have added ourselves we will need to flash the OS onto it.
No. You must flash one of Omega2 Pro firmware onto the Omega2S+ onboard flash.
For example if you want to use the latest Omega2 Pro FW (2019-10-28) then flash it onto an Omega2S+:
sysupgrade -n --force omega2pro-v0.3.2-b232.bin
The full firmware upgrade process may take up to 5 minutes and your Omega2 Pro will reboot twice!
The first reboot will happen after the firmware has been updated, then once the device initializes, it will configure everything to run from the eMMC storage, after which it will reboot again. This will only happen on the first boot after an upgrade.
It'll take about 7 (seven!) minutes.
You should not press any key during this time - it is fully automatic.
You could watch the whole process on the Serial Console.
BTW The title of this article Booting from External Storage is a marketing bullshit.
An Omega2(+), Omega2S(+) or Omega2 Pro - with any present official Onion firmware - is not able to boot from any external storage (MicroSD card, USB memory storage or eMMC).
Please take a look at my project: DIY Omega2 Pro - Try it before buy
Unfortuntely I don't have any Omega2S+ and I can't solder any BGA chip - so I've made some experimental (or "fake" ;-) Omega2 Pro for myself.