headcanon stuff, since you got me thinking about oz and buffy's dying and coming back to life; how d'you think oz would have initially taken her death and processed it if he'd still been with the scoobies during that time?
can i just say, i love you for asking this.
this is where i think it is important that oz was not around in the scoobies life during this time, because he absolutely would have stepped in and told them that they have no right meddling in the forces of life and death. he would have been adamant that he was not taking part in it. he would have told willow and tara and xander that what they were doing was reckless and playing with magic that they did not know or control.
the fact of the matter is, giles didn’t believe that buffy was in another dimension. nor did angel. both of whom had been doing it a lot longer than willow, xander and tara had been involved in these things. he would have told them that angel did go to an alternate hell dimension, he took his body with him. buffy was dead. not in a different dimension. she was dead.
he would not have had any part in it, he would have told them that they’re dealing with dangerous magic. which i think he would have been looked at with dark expressions from all three of them. oz has always been the voice of reason so having him there would have put a damper on things.
unfortunately, he has no control over tara or willow or xander. he doesn’t want to. he would have voiced his concerns. and he would have taken a step back. hoping that they would have listened to him. when they didn’t oz was just as furious as giles was. he had warned willow of the dangers of power you don’t know or can’t control. his warnings went unheard and he would have probably realized in those moments, how little he probably meant to the rest of the scoobies.
but lets talk about what you actually asked. oz would have been devastated at buffy’s death. he would have even probably broken down at some point. probably in silence. willow and tara had themselves to console in. same with anya and xander. the only others he might have leaned on would have been giles, or spike. because he is so stoic and quiet, people would have taken it that he had dealt and grieved in his own time and was the strongest of them all. but it was probably very likely that he would have broken down privately. or even probably only in front of giles.
buffy above all else was kind to him. if oz had stayed, it would have been likely that their friendship grew. we saw the first start of that happen in season four before he left. they were not close but they could have been. had oz stayed, or even returned to sunnydale after his time away, he would have definitely become a confidant that buffy would go to. because she would have known she could trust him and his silence. oz always was very protective of buffy as well. he was also older than the other scoobies by a year and always felt very protective of them in a way. he was always the outsider looking in but once he and willow broke up and willow found tara, he found new ways of making himself useful to the group. because these were his friends.
losing buffy is like losing the matriarch of the gang. oz would have internalized a lot of his anger and probably retreated into himself and the wolf a lot more so that he didn’t need to think about it. oz is someone who handles things on a very mature and emotionally grounded way. but losing buffy might have cracked that resolve he has. he cares very deeply for her. probably more than buffy ever expected if honest.