Wow 20 is an awesome number! Let's hope they are all good ones and you get lots to the embryo stage.
I'm actually gate crashing this post a bit as I'm not going through ivf at the moment, I did it 2 years ago and now have a gorgeous 16 month old to show for it.
In terms of my ohss, my case was really unusual so my symptoms were different. Instead of the fluid building up in my abdomen it built up in the small space between the lung and body wall (pleural space) so my symptoms were initially shortness of breath and later on I had very bad abdominal pain right underneath my ribs. Although my ovaries were very big I didn't actually have any pain or obvious bloating. So due to lack of ordinary symptoms and low risk (I had 12 eggs collected so not huge number) the doctors didn't even think about ohss at first and were treating me for a blood clot in my lung. It took 4 days and a lot more fluid before they diagnosed me and in the end I drained 24l of fluid from the right side of my chest over 3 weeks. Not pleasant!! As I said before the worst part was that as I was pregnant the doctors just kept saying I had to wait until things settled down which potentially could have been 12 weeks. Luckily it was only 3 weeks in hospital but that was bad enough! Also you don't really need the added stress of all the tests, xrays, medications etc while you are pregnant and worrying it might affect baby.
Hopefully if the nurse is happy then they are not too worried about you and mild ohss is very common. As I've said just stay hydrated and try to eat protein as that can help reduce the fluid build up. I'm hoping your embryos all grow nice and strong and you get a great result first time. X