I have struggled with the EXACT same situation since returning to work almost two months ago. LO settled in at 14-16oz. everyday while I am gone. I have tried Oatmeal (definitely helps), fenugreek (might help a little), adding a third pumping session at work (didn't do a thing for me), different size shields (might get an extra 1/2 oz) etc.
I was SO stressed about not pumping enough during the day. For me the best thing I did was start pumping in the middle of the night. Milk supply is highest then and LO STTN very consistently. So, I pump 7-9oz. while I'm at work and 4-5oz sometime between 1-3am depending on when LO and I go to bed. I do this EVERYDAY, even the weekeends that way I start the week with at least an extra 10-12oz in the fridge that can be used throughout the week if I fall short a couple oz. everyday. I've also been lucky enough to work form home a few days over the last two months and that has allowed us to get through a few tough weeks without having to dip into a very small freezer stash or add formula. Infact, and I've even been able to replace a few bottles of my freezer stash with the nightitme pumping and days I worked at home.
So it is hard work and a defintie sacrifice of sleep for me, but I've made it work. When I first went back, my Mom, who watches her was slightly over feeding her, but we addressed that and she still takes 14-16oz. DD is a tiny little thing (at 4 months she was only 11lbs.) so although overfeeding can be an issue trust your gut, I know my DD and overall she is predictable for a total amount, but how she chooses to do it can vary from day to day...Some days she eats two 7oz feedings while I am gone, sometimes three 5oz feedings and sometimes, four 4oz feedings
I will be honest and say I don't know how much longer I can keep up on this schedule. I hope that as we add solids towards the end of the year she will reduce her need by a few oz a day and I can stop having to pump at night...If this doesn't happen I may start to supplement with a few oz of formula when she is around 8-9months so that I can start getting some better sleep.
Best of luck to you!!