Character Analysis
Oldest and Most (Self-)Important Sibling
Osbert's the oldest of the siblings, and in typical oldest fashion, spends most of his time "looking superior" (1.3.5) and saying things in a "slightly sneery way" (1.3.12). Of all the people Daisy encounters in England, he's the only one who "reminds me of people I knew in New York City" (1.3.12). And given how glad she is to be rid of the States, it's safe to say she doesn't mean this in a good way.
Although he's only a year older than Daisy, Osbert doesn't hesitate to remind her at every turn of this precious age advantage. The dude's "always pushing himself into the conversation with superior information and making it clear that he was the one with Family Responsibilities Which Frankly Exhausted Him and He'd Rather Not Be Bothered only Seeing As How He Was the Eldest, Well, deep sigh" (1.8.3). Sounds like a really fun guy to be around, are we right? Meh.
Wannabe Super-Spy
And if acting all-important about his role in the family weren't enough, Osbert has also deigned himself War Expert along with his schoolmates. He loves to talk about how "it would be amazing to live in London and be spies and duck around the enemy to get information" (1.9.20), though we're on Team Daisy in realizing that "Osbert and a bunch of his snotty schoolboy friends" (1.9.20) aren't exactly first on the list for the prime minister to call in a crisis.
The Protector
Though Osbert likes to remind everyone of his supremacy in the family, when faced with actual authority figures, he's subservient and eager to please. When the soldiers arrive to take over the house, "Osbert was so anxious to look helpful he was practically standing at attention" (1.14.4). Lot of good that does, but it's sort of sweet that Osbert's "hoping that somehow he could protect us all by being Respectful" (1.14.4). His protective instincts reappear later in the book, post-war and post-death of Aunt Penn, when he adopts his younger siblings.