There's a bearing and seal on each side of the ring gear and there's a bearing and seal in the brake drum cover. This doesn't include the bearing and seal in the end of the axle tube on the left side of the axle. The o-ring is supposed to go on the axle. In your pic, I can see the old one between the brake drum and the splines.
Most 4-wheeler problems are caused by a loose nut connecting the handlebars and the seat!!
You only need two tools in life -- WD-40 and duct tape. If it doesn't move and should, use the WD-40. If it shouldn't move and does, use the duct tape.