Sasha Pastujov had a goal and an assist.
Cam Allen, Matthew Papais and Braedon Bowman also scored for Guelph, while Joseph Serpa and Roman Schmidt scored for Kitchener in front of nearly 3,200 fans at the Sleeman Centre.
It was Oster’s third win of the season, having played four games so far, as he shares the crease with Owen Bennett.
The 17-year-old Oster also made 35 saves on Friday, when the Storm beat the Sarnia Sting 2-1.
Guelph’s record now stands at 3-3-1. They will next begin a series of three games in three days, beginning with a trip to Windsor to take on the Spitfires on Thursday.
Sunday’s loss was the Ranger’s first of the season. They are now 5-1 through the first six games of the season.
Next up, Kitchener is on the road on Wednesday, taking on the Sault Ste. Marie Greyhounds.
Guelph and Kitchener will renew their Highway 7 rivalry on Nov. 2 at the Kitchener Memorial Auditorium.