Not really,,,,,More likely there is a double edge. IE a negative edge going into the micro from the opto coupler is undershooting and bouncing back above the logic zero threshold level. The result the that the micro sees the rising edge of the ringing. It is very possible that the ring may not have been visible on the O-Scope if the scope sweep speed was too slow, or the scope had too low of bandwidth, or the scope had BW limit turned on, or the GND lead of the scope was connected in the wrong place, or the GND lead was not hooked up or too long.

By the way, what opto part are you using? Maybe a different opto may be in order. It is possible that IF the opto is one of the high bandwidth types with the active outputs the part may actually be passing a ring that is occurring on the input side of the part. I have also seen issues in some buffered types of parts where the output acts wierd if you under drive or over drive the input diode.

Michael Karas