ALTDEV ALT OVERSHOT DURING CLBOUT.

Narrative

WE LEVELED OFF AT ASSIGNED ALT (5000 FT). ORD DEP THEN ISSUED INSTRUCTIONS TO CLB TO 6000 FT (A SOMEWHAT UNUSUAL CLRNC AND ALT FOR O'HARE). I READ BACK 6000 FT AND DIALED 6000 FT INTO MODE CTL PANEL ALT. I NOTICED TFC ON TCASII AT OUR 2 O'CLOCK AND ABOUT 5 MI; AT 6900 FT; AND WAS LOOKING FOR HIM OUT MY WINDOW WHEN I HEARD THE CAPT DECLARE (EXPLETIVE DELETED) AND LOOKED BACK IN TO WATCH HIM NOSE OVER AS WE CLBED TO 6400 FT. HE DSNDED BACK TO 6000 FT. I SHOULD HAVE BEEN MORE DILIGENT IN WATCHING HIM CLB OUT. HOWEVER; HE IS AN EXCELLENT PLT AND HAD NEVER PREVIOUSLY GIVEN ME CAUSE TO 'WATCH HIM!' ALSO; HE ELECTED TO USE VNAV FOR THE CLB; EVEN THOUGH IT WAS ONLY FOR 1000 FT. VNAV CLB WILL GIVE FULL CLB PWR AND SOMETIMES AN EXCESSIVE CLB RATE (3000-4000 FPM); AS IT DID IN THIS CASE. I SHOULD HAVE INCLUDED THE INSTS IN MY SCAN EVEN WHILE I SEARCHED FOR TFC. THIS WAS THE CAPT'S FIRST TRIP BACK AFTER A LONG LEAVE WHICH ALSO HAS ME WONDERING WHEN; AFTER SUFFERING SUCH EMOTIONAL TRAUMA; IS IT OK TO RETURN TO WORK? YOU CAN'T STAY AWAY FOREVER; BUT HOW DO YOU KNOW IT'S OKAY TO COME BACK UNTIL YOU DO SO? I THINK HE'S DOING FINE; BUT I COULD TELL HIS ERROR UPSET HIM. AGAIN; IN LIGHT OF ALL THIS; I SHOULD HAVE BEEN MORE DILIGENT.