1874: Oscar is born, the last of the Daphnel children.
1875-1884: Childhood proves largely uneventful. The family lives in the countryside and occasionally travels to academically-interesting destinations. Ozzy is a happy, if attention-seeking, child, and demonstrates magic in a fit of pique after Jasper goes to Hogwarts. Luckily, he only has to wait a year after Beatrice goes to school to go off to Hogwarts.
1885-1889: Oscar goes to Hogwarts and is sorted into Gryffindor. He is a bright student who could be better at his studies if he was less focused on charming his classmates; as it is, he has a large friend group and generally enjoys school. He is distantly concerned for Bea, who does not "thrive" at Hogwarts and is eventually placed in Pendergast. Oscar, as a bit of a class clown, is not made a Prefect.
1890: Following his OWLs, Oscar takes after the tradition established by his eldest brother and his father, and takes a LOT of N.E.W.T. classes. This is largely because his biggest ambition is to be wealthy and interesting, and while he's certainly the latter, he will need to make money or attract investors to do the former.
1892: Oscar finishes school with an assortment of N.E.W.Ts and spends his summer faffing about and waiting for the results. Once he gets his grades, he applies to and becomes an Apprentice Cursebreaker at Gringotts — because maybe adventure can play into his eventual financial schemes, right?
In November, Victor marries a Miss Christabel Dempsey. Oscar starts to consider moving out of the house — the married couple and Bea's debutante life is harshing his vibe — but then Bea ushers in the new year by accidentally killing Victor. So, Oscar puts moving out on the back burner.
1893: Victor becomes a ghost, and the Daphnel siblings fake Victor... not being murdered by Beatrice, but still dying, because obviously he is dead. Oscar tries not to be too stressed out about the whole thing, because someone alive has to hold it together, and it's obviously not going to be Jasper or Christabel. (Learning to be a curse-breaker is going well, by the way.)