I scrolled down study organizer and noticed 'Extra' in week column. I'm just wondering whether I should do those OG problems after I completed the nine sessions or whenever I want? I'm currently in my first week of the online nine sessions.

Save the extra stuff for later - for now, stick to the main two-page syllabus that you were given during your first class. As you get closer to the end, or after the course ends, you can do the extra material if you like. Right now, though, concentrate on the main stuff.