Barnsley FC boss proud of win, but can't control what Sheffield Wednesday, Plymouth and Ipswich do in race for promotion
Tedic scored the goals either side of the interval with Adam Phillips providing the assist on both occasions.
The Reds have now won nine successive home matches, while struggling Oxford are only outside the relegation zone on goal difference and are without a win in 17 games.
On the performance of Tedic, Duff said: “He’s been excellent. We’ve watched him grow during the season.
“He’s definitely a different animal now compared to when he first came here.
“He came back from injury, physically miles better. Manchester City don’t pay three million euros for someone they don’t think is very good.”
Duff, whose side remain fourth, added: “I thought it was a good professional performance, a little bit scruffy at the start but we scored at a good time.
“The second half was as comfortable as you can get, I thought we dominated the second half.
“It’s tough to win games, there’s an expectancy now.
“We were much better with the ball today, we hurt them in lots of different ways.”
On his side’s strong home form, Duff added: “It’s an unbelievable achievement.
“Word got out a long time ago that we’re not a bad team, so teams are coming in to be difficult to beat and we’ve found a way every time.”
Despite the league’s top three sides all winning, Duff said: “I’m not delighted they’ve all won but I can’t control it. We’ve done our job and they’ve all won – fair play to them.”