Kevin
I have fished Okangan Lake for the past 18 years, mostly the south end of the lake from Rattlesnake Island to Penticton.
TO catch the big ones -
You must have a downriggger.
Try to locate the thermocline, and fish within a band of about 20 ft including the tthermocline. The big rainbows tend to feed on Kokanee up to 1/2 lb or so and Kokaneelike warmer water than Rianbows. In summer they will be below the thermocline and the Kokanee will be above it.
Troll at approx. 2.5 - 3 MPH with Apex. plugs, and spoons. A short hook up of about 12 - 15 ft back from the downrigger cable seem to be more productive.
Best time for me has been just at dark in the evening.
In winter, try bucktails,or spoons on top, with a fast troll up to 5 MPH
You should know this can be a very frustrating experience, in Okanagan Lake with many boring hours, without a bite, but I have landed many fish in the 10 - 15 lb range with a few in the 15 - 25 lb range. Well worth the effort. A big Rainbow in Okanagan Lake is like catching a big Spring Salmon in the Saltchuck. My boat is the Kingfisher, a Trophy 205e out of Penticton. Try me on VHF Channel 16, ndight lines!