I'm Europe-based who frequents OS both intra-Europe and TATL, and I'll introduce my data point, which is probably in the minority. Basically, what seems like almost 50% of the time, my OS flights don't automatically credit to my UA MP account, and I find myself needing to wait the 15 days. I can't tell you whether in those instances where the miles weren't posting automatically were on tickets bought via UAL, AUA, or an OTA, or whether they were on codeshare flights. Generally speaking, OS has not been 100% reliable when it comes to posting when I fly on their metal. Also, I don't know if my experience is a problem within OS where one might have a similar experience if their OS flights were being posted into M&M.

I really enjoy flying OS, except when it comes to waiting for their flights to post.

Also, fly on UA metal 4x. It's so easy.