I love it when you find siblings with combination names or cluster names, not really sure what else to describe them as.
I went to school with a boy called Adam whose elder brothers were called Christopher and Stephen.
Then I dated a man (Peter) whose three brothers were called Adam, Christopher and Stephen.
Now my DH has two brothers and the three of them together are Adam, Christopher and Stephen (not saying which one is my DH).
But there must be something about those names that just "go" together, other than that they are traditional names that were popular in the 70's/80's.
It seems like too big a coincidence to me that I know three families who have used all those names together for their sons.
I'd guess that Oscar and Jacob have a sibling named Thomas, Ollie or Max.
And my shadow name, had I been a boy, would have been Rogan. Rogan with possibly Josh as a middle name, because my 1970's mother had heard it well before she realised it was the name of a bloody curry. My brother, had he been a girl, would have been Leona, which for some reason my mother decided against saving for me. I have a feminine version of my grandfathers name (the same grandfather I decided to name LO after).