Taylor Harwood-Bellis and Eric Garcia were partnered in the heart of defence against the U's.
And their performance left an impression on Guardiola.
"With the second balls and throw ins - Oxford are incredibly tall and are good at set pieces," Guardiola said. "In these kind of leagues, they work well in these situations.
"But Eric and Taylor contained the attacks incredibly.
"It was really tough. Winter is here! They are aggressive. Lasts season they played good and this season they were better.
"Cup competitions in England are always really tough."