it is not only depends on the type of signal I mean analog or digital but optical isolation also depends on the frequency of the signal....there one need to understand little more about your system.....For an example did you planning to have polling based system or interrupt based system too.... so first write down the specification of your system then one can put the ideas.....Opto-coupler in general will give you millisecond kind of resolution and precision in the digital signals.... but if you look for system specification in microsecond it may not be good choice how ever higher quality and costly opto-switching technology in today gives GBPS support in the fiber optics....
Good Luck