Ritchie is still hero worshipped by Oldham fans after his exploits as a player but his Huddersfield side were blown away by three goals before half-time.
Jason Jarrett scored twice and Leon Constantine was also on the mark as Oldham hung on to an outside chance of reaching the League One play-offs.
Chris Taylor bagged the fourth for a Latics team who could have hit at least half a dozen before Jarrett blew his chance of a hat-trick when his penalty was saved by Matt Glennon.
Ritchie was so upset by Huddersfield's performance that he kept his players in the dressing room for 45 minutes and later said: "It was an absolute disgrace and embarrassment.
"The players should be ashamed of themselves because they did not even put up a fight."
Huddersfield had missed the first chance of the game through Andy Booth and their misery began when Oldham made the breakthrough on seven minutes.
Constantine set up Deane Smalley for a shot which came back off the crossbar and was bundled in by Jarrett from six yards.
Defender Stefan Stam had an effort cleared off the line and dominant Oldham doubled their lead after 40 minutes following a bad mistake by Frank Sinclair.
The veteran's first touch let him down and Taylor raced through the middle before slipping a pass to Constantine, who netted with ease from eight yards.
Mark Allott also went close and Jarrett made it three on the stroke of half-time when Nathan Clarke was penalised for handball and the resulting free-kick was slammed into the top corner from 20 yards.
Huddersfield carved out a rare chance which was wasted by sub Malvin Kamara, but then Smalley crossed for Taylor to score from close range at the far post.
Glennon kept out Jarrett's penalty, diving away to his left, and the rebound was fired against the woodwork by Allott.
Huddersfield's scant consolation came in injury time when a combination of David Mirfin and Oldham defender Reuben Hazell helped the ball over the line from a deep free-kick.