Can anyone point me in the direction of the memo or ALARACT stating what can and can not be worn with OCPs?

I have you a broken link, but there's plenty of options on the interweb

1 July 2015 to 30 September 2019 is the uniform transition period. In this window, Soldiers will wear combinations of the three uniforms above, as indicated, until the mandatory possession date of 1 October 2019 when the entire Army will be in the Operational Camouflage Pattern Army Combat Uniform.

I thought it was authorized to October 2019. So FY20