Here we are told that Oliver met Lisa when his plane was shot down over Hungary in World War II and she was a member of the Hungarian underground, whereas in Season 2, Episode 8, Oliver describes how he and Lisa met on a luxury ocean liner where he was a musician for the ship's band and she was the daughter of the ship's owner.
When Oliver ejects from his aircraft and lands in the tree, from the story of how Oliver and Lisa met during WWII, Oliver tells Lisa he's with the United States Air Force, But the US Air Force was not formed until 2 years after WWII, making it still the US Army Air Forces, or US Army Air Corps, depending on when he served.
There is a reference the fact Hungary remained an independent country during the entire War, but, like most of Germanic Europe, Hungary did became ruled under Nazi Germany.
Hungary was an ally of Germany's during most of World War 2 and thus was never occupied until the Soviets occupied it in 1945. There was no Hungarian Underground movement. There was political opposition to the government's choice of allegiance but it was not armed. German tanks did not patrol Hungary as it was their ally.
Around the 12 minute mark, during a WWII flashback, after blowing up a tank, Lisa says they blew up twelve tanks the day before while she wore a bikini. While two piece swimsuits for women existed at the time, the "bikini" was invented and given the name in 1946 and named for the atomic test at Bikini Atoll.