Is is advisable to buy a ticket for this museum online before arriving in Paris or is it possible to decide what day you want to go once you are there?
I bought tickets two years ago online and they were not date specific. I just went on the the day I wanted to go.
Why not?
You get to enter through a reserved entrance so you will not probably have to stand in line to buy a ticket
you could buy it online on a the hotel computer when you get there also
and FYI first Sundays of the month are free to enter but crowded
but if you are visiting more than the Orsay, it may make sense to get the museum pass
You can buy a ticket from the newstand out front when you arrive -- never seen a line for that -- and then use the ticketed entrance.
Buy your ticket in advance, then look for Entrance C - it's around the far corner to the right, and is for advance ticket holders.