Should I use plywood or T&G OSB for subfloor in new addition I am
building? The addition is on second story, floors are 2 X 8 16 O.C. I
checked prices and 3/4" T&G OSB is more than twice cheaper then 3/4"
plywood. I will have hydronic heated floors. There will be another
plywood or OSB subfloor on top of base one with grooves for PEX
tubing. My original idea was to use OSB everywhere except for bath
subfloor where I was going to use plywood.

They're basically interchangeable. Check the span ratings stamped on the
sheets to see what would work for your application. Standard plywood is
slightly stronger than OSB, which means you might need to use a thicker OSB
for the same application. But it really depends on the usage. In many cases
you can use either in the same thickness.

I used 3/4" T&G OSB for the subfloor of our house and it works very well
(16" OC joist spacing). I've heard it can be slightly more sensitive to
moisture during construction, but our subfloor was exposed to the weather
for a few months while we built our house, with no signs of swelling
anywhere. But we did sweep off the subfloor each day to remove any standing
water. If you think it will take you a long time to build, plywood might
be a better choice (or just cover everything with plastic).

Another thing to consider is weight. OSB is a bit heavier than plywood, so
I used plywood for our roof sheathing. It's not a huge difference, but I
appreciated the lighter sheets when I had to hoist them up to the roof
solo.

I used OSB sheathing for the walls of a remodel at my in-laws, since it was
being covered with siding anyway. For our own house we wanted a rough-cut
face which was only available in standard plywood at the time (though I did
buy an OSB version later for our shed).

I generally choose whichever is available at the lowest price that meets
the span and appearance ratings I need. In most cases, OSB works fine.

Anthony