Educational Snobbery
One of the central themes of the novel is educational snobbery. Oxford University views itself as the benchmark of educational excellence, but the faculty are also without the common sense to recognize that Paul is the same person who had been expelled and not his own distant cousin with the same name. The importance socially of having an Oxford degree is also emphasized as Paul is only entitled to his inheritance if he graduates from Scone - no other college degree will suffice.
People Are Not Who They Appear To Be
People in the novel are often not who they appear to be. Paul is obviously an example of this, returning to Scone as his own cousin. However, Margot is also not the person portrayed on the outside. She appears to be an upstanding and morally righteous member of high society but is actually running a prostitution ring.
Social Confusion
Waugh wanted to lampoon what he saw as social confusion of the 1920s, a time during which the so-called upper classes behaved in a manner that had previously been thought debauched or immoral. The confusion around what was actually moral and what was not is the thematic cornerstone of the book. Waugh pays particular attention to the fact that there are many different moral sides to each of the characters and also that they are the same people regardless of the current vogue in morality.