Oregon is one of only a handful of states with what is called "transactional" immunity. Most other states have a "use" immunity system, most notable in the prosecutions in the Watergate and Iran-Contra prosecutions by federal prosecutors.
The difference is simple, but drastic.
The Fifth Amendment provides everyone in America, whether a citizen or not, with the absolute right not to give testimony against him/herself. Police and prosecutors sometimes may try to solve a perplexing crime by offering a person who may have knowledge of the crime "immunity" if that person will tell what he/she knows. Of course police and prosecutors don't want to give such immunity to the main evil-doer, who could then fully confess and avoid any responsibility.
"Transactional" immunity is an all-or-nothing proposition. Under Oregon's current scheme, a person given immunity to testify can never be prosecuted for any crime related to his/her testimony, no matter what he/she has said or admitted. One typical situation is a rape case in which two men in a group of six are suspected of the sexual assault. None of the men are willing to speak and it is unclear who actually raped the victim and who may have actively helped. Under Oregon's "transactional" immunity, anyone made to testify can never be charged with anything relating to the rape, even if other evidence or eyewitnesses later become available.