I haven't worked with SPOs yet, but I do work frequently with decisions, so here's my 5 cents...
In your case you wish to have your decision based on the data contained in "a delta load to the DSO". I'm assuming you're talking InfoPackage here, because you wish to execute the DTP's afterwards.
So basically you need to determine which "months" are in your incoming data packages. The only way I see that possible is via start/end routine. In your example, you would look the data package and find 3 records each with a different month, being 08.2014, 09.2014 & 05.2013. You could then "convert" these to just numbers ranging from 1 to 13. That makes it relatively "easy" to link the right DTP to the right "number".
Now, how to pass that info (those numbers) back to the "decision" process? By default you base your formula on a "system" field (see basic how to link below). I don't see how this would work in your case. You'll need to somehow export your "numbers" (most likely an internal table or a "range") to memory (or fill it in a [z-]table) and then read it back in with custom code. I haven't done that yet (at least not in the context of decision steps), but I would recommend to have a look at the "experienced" way of working document below. It won't be easy, but I think it could work.
Cheers,
Raf
The official documentation can be found here.
For a "basic" how to on decision step, click here.
For a more "experienced" way of working, click here.