All can be known with the digital short novel - HALO: NEW BLOOD!
Anyways Fire Team Osiris are not bad guys. They aren't HUNTING Master Chief to kill him, they are trying to find him and bring him back home to the UNSC. In New Blood Buck is recruited into the SPARTAN IV program by Jun-A266, along with the rest of the Alpha Nine Team (Rookie, Romeo, Dutch, Mickey.) but yeah won't spoil it for you. Just that after New Blood he wanted to be apart of a Team but not lead it this time around.
as far as i can tell, he isnt neccesarily the bad guy, as we dont know much about the campaign. he is just sent to track down and capture the awol blue team with the the rest of fireteam osiris. so possibly good guy
Nobel I don't have an xbox one and yet I'm digging through untold amounts of crap trying to get a small amount of the story solved and guess what its a big cliff hanger. the bad guys aren't the chief or lock its the same guys from halo 4 and that elite has been given command of forerunner mega weapons, but I think the chief has one of them under his command.