Slaven Bilic hailed his West Brom players after Albion returned to the summit of the Sky Bet Championship.
They need eight points from their final four games to end their two-year exile from the Premier League.
Derby had Louie Sibley sent off late on and missed the chance to close the gap on the top six.
Second-placed Leeds host Stoke on Thursday and will return to the top with a victory but Bilic was proud of the Baggies on Wednesday.
"The guys below are putting the pressure on us and Leeds, us and Leeds don't put the pressure on each other but it's nice to be back on the top of the table," he said, with the Baggies five points clear of third-placed Brentford.
"We were magnificent, we were fantastic. We played with a very offensive team and to defend like this against a good team and give them nothing and keep the ball, create so many chances – the only thing which was missing was an earlier second goal.
"I'm so proud of them. We had very offensive players but the way we defended was great. Everyone did the job. To leave Derby in this form they are in with nothing requires not only patience and quality, everything was there.
"It's not only three points, the performance is giving you belief and confidence."
West Ham loanee Diangana opened the scoring after 11 minutes when he netted from a tight angle after latching onto Kamil Grosicki's pass.
Matheus Pereira remained a constant threat and shot wide while Ben Hamer denied Charlie Austin.
Albion never looked in danger and O'Shea wrapped up the points 14 minutes from the end, heading in Pereira's corner.
Derby remain three points adrift of the top six and Sibley was dismissed in injury time after a foolish kick on O'Shea.
Boss Phillip Cocu said: "That is no red card. I think we will appeal. I don't know if you saw the footage from close range, at first I wasn't sure as it was on the other side of the pitch.
"But when I saw the clip it was 100 per cent no red card. If he gets a suspension, for me, it would be unbelievable."
Cocu also empathised with Wayne Rooney after the former England international failed to make an impact.
He added: "I know what he (Rooney) was annoyed about, the same as we were all annoyed about – that is how the referee was leading the game, or not leading the game. A lot of decisions were very doubtful.
"Everybody can have a good or bad day, like us as players or managers, but sometimes you also have to take responsibility if it was a nightmare."