Theoretical question deal with topics that do not generally have immediate practical uses. Please be careful when using this tag: your question may be more appropriate for the Computer Science Stack Exchange site.
Questions about "algorithm and data structure concepts" are on-topic here. Questions about "algorithms, models of computation, programming language semantics, formal methods" are on-topic at Computer Science SE.
They key difference is if you are doing a "mathy" proof and evaluating Big-Oh for the sake of evaluating Big-Oh, ask at CompSci.SE. If you need help designing an algorithm and the math or Big-Oh is simply a means to an end (a working program), Programmers is most likely a better fit.