It seems to me that most theories involve postulated objects, and then various laws that describe how those objects must or can relate to each other. So, you might postulate an id, ego and superego, or genes, or electrons, protons and protons, etc.
It also seems to me that there are at least two types of "simple" when talking about explanations. There's a brevity "simple" -- like a maths proof or a piece of computer coding with minimal steps. And there is also an ontological "simple" -- an explanation relying on as few postulated objects as possible.
If it's true that there are at least these two types of "simple", well, does that render parsimony often difficult to apply, if you're committed to it as a good rule of thumb when deciding what to believe in? One candidate theory could be ontologically complex but brevity-simple, whereas the alternative theory might be ontologically simple but convoluted.
Here are some things that worry me: (1) does appealing to deities lead to simpler explanations that ones that don't involve deities? Does OCcam's razor cut in favour of gods or against them? (2) philosophers are interested in all sorts of things that apparently aren't physical -- like universals, concepts, propositions, meanings, mental states, laws of logic. If one is commmitted to physicalism (on the basis of ontological simplicity), should one endeavour to be eliminativist about anything you can't jab with a stick, even though you can land yourself in explanatory difficulties by trying to do without these notions?