Online master's degree in CS at the Georgia Institute of Technology
In my opinion, Georgia Tech's Online Master of Computer Science will not help you get a computer science PhD in the US.  Not because it's online, but because it's a professional master's program.

There are two types of master's degrees in computer science in the US:


Thesis (aka research) masters degrees have a significant research component, in close collaboration with an advisor, leading up to a thesis.  These are generally seen as useful stepping stones to a PhD.
Professional (aka class-based, aka "taught", aka terminal) masters degrees consist entirely of classes.  These are generally not seen as stepping stones to a PhD, because there is no expectation of (and therefore no resources for) research.


Moreover, because of the huge scale (>3000 students) of the OMCS program, having any kind of sustained on-on-one interaction with faculty—which you would need to get strong recommendation letters—is almost impossible.

(I regularly serve on the graduate admissions committee in a top-10 American CS department.)