OSIRIS is used for course administration. It is also the only official channel for registering student grades. You can use the gradebook in Brightspace, but only if you also register the grades in OSIRIS.
Each academic year OSIRIS is used to designate educational personnel and students to courses. Based on this information lecturers, colleagues (Observers) and possibly student assistants will be enrolled in the right Brightspace courses (Course Offerings). Please read the article Which roles are available in Brightspace? for more information about which OSIRIS role is linked to which Brightspace role.
Based on their OSIRIS enrolments students will be automatically enroled for the corresponding Brightspace courses. They will also be automatically assigned to a group within a course when you have indicated that the groups need to be transferred to Brightspace. Read more about (un)enroling for courses and groups here.
For more information about the relationship between OSIRIS and Brightspace, please read the following article: How is a course created in Brightspace?