You don’t have to plan from scratch. These rides run through Oak Bay or pass close by, mapped and written up by route publishers around BC. The full maps and turn-by-turns live on their sites.
Seaside Loop— Let's Go Biking · 40 km route, 7 km of it through Oak Bay
Touring Victoria— Cross Canada Cycle Tour Society · 84 km route, 6 km of it through Oak Bay
Metchosin Caper— Bike Gear Database · 83 km ride, passing within a kilometre
Vancouver Crossing Loop— Adventure Cycling Association · 503 km route, passing about 1 km away
Victoria— Let's Go Biking · 20 km ride, passing about 1 km away
Getting there with a bike: province-wide buses, ferries, and shuttles are listed under Transport (some serving Oak Bay may be listed there rather than here).