Re: Do I have oversupply?
well, the 'rule of thumb' is that breastfeeding works on supply and demand. your body adjusts the supply to meet the demand.
Therefore, if you're pumping more than your baby can drink, then yeah, it's a slight case of oversupply.
If you cut back a little on your pumping schedule, then after a short time, your supply will drop back to match it.
As long as you are consistently pumping more than you need and building a "stockpile", then it wont hurt ot cut back a little.
on that point, dont forget that the frozen milk doesnt keep forever, generally 3-6 months, depending on the temperature of your freezer. (In the nursing mother's companion it states that milk can be kept for 6 months or longer only if it is stored below 0 degrees Farenheight. So if you're finding that you just build up a stockpile of unused milk, it might be a good idea if any of it starts to get really old to use it and freeze the more recent milk.
You might try stretching your pumpings to every 3 1/2 hours and see what happens, or, on the other hand, if it's no problem for you to collect excess, it can be nice for a rainy day. you might even find a mother struggling to get enough milk who would be blessed to get a donation of some excess milk.
Re: Do I have oversupply?
H Erin, Your body will respond, as the PP said, to your demand on it for milk. If you cut back on your pumping a little, that may help. Your supply shouldn't drop drastically - but if you find that it drops more than you want it to, then you can adjust and fine-tune your pumping schedule. Keep in mind that it may take several days to a week or more to really see changes resulting in supply.
Another thing to keep in mind, though, if you change your pumping schedule, is to keep a close eye on the health of your breasts! By this I mean that you don't want to have plugs build up. So be sure to check your breasts when you pump them. Feel all the way through them. If you hand-express to help clear milk (always a good idea if you can) after you are done with the pump, this is a great way/time to check for any changes in texture, consistency, soreness, etc. and to make sure that you really are getting good emptying (a misnomer, of course; your breasts are never totally empty while lactating) even with the decreased pumping schedule. This hand expressing shouldn't stimulate another letdown so it's not at cross-purposes with your overall goal.
Also....if you have room for all that extra you are pumping and keeping, you may just want to have it! Never know what may happen a few months out, etc., etc. and you may wind up being thrilled to have a fall-back supply. (I had over 200 bags at one point and am so happy I did - now I'm down to 20!) It's more work to coordinate, but not a ton, so do be sure to rotate your stock - like the PP suggested: use some of your frozen milk each day and freeze some of your freshly pumped milk so that the older frozen supply doesn't go to waste!
Just some more thoughts.........good luck! :)