I guess it hinges on the scope of the "form and legality" review. There is an obvious conflict with the law that bans immigration rules based on country of origin. Did the OLC weigh that (or even constitutionality)? Or merely whether the preceding law that Trump's order cites, authorizes him to give the executive order?

I assumed the latter. We'll soon see how the legality of the order plays out in the courts anyway.

https://www.nytimes.com/2017/01/27/opinion/trumps-immigratio...

Edit: Either way, my original blanket assumption that it had just not been looked at by the DOJ at all is clearly false as the OLC is a part of the DOJ.