I much prefer Robert to Oscar or Sebastian and there are so many nn for Robert. Your son will be able to choose which he prefers at any stage of his life. My brother has a very good friend called Robert who is generally called Robbie but some very close friends call him Bertie so your post made me think of him.
Rob Bob Robbie Bobby Bertie Bert
Does a name have to be exciting? I like good traditional names and you are choosing a more unusual middle name.
I love Robert and, as others have said, it has loads of nickname options - I love Bertie and Robbie for a little boy and Rob for a teenager!!
Don't mind Sebastian but really don't like Seb, especially not Seb Duckworth.
Oscar is ok, but very, very trendy at the moment. I would steer clear of such popular names as they are likely to 'date' more than a classic name as Robert. Also there'll likely be others with the same name in his class/school.
I'd stick with Robert. But go with what you and dh love most!!