What is the quickest/most efficient way to get from SSR to OKW?
Walking is certainly an option.
I was just using the measure path tool in Google Earth to measure some paths. I don't know which section you're staying in. But here are some sample walking distances:
It's about 1.31 miles to walk from the SSR Carriage House along the river to the OKW Hospitality House. In the scheme of things, and the amount of walking you do at WDW, a mile and a half isn't that far. Depending on your pace, walking is probably the quickest way to get there.
From the bus stop at the Grandstand Section of SSR, it's a 1.21 mile walk to the Hospitality House at OKW.
From the Treehouse villas section of SSR, it's a .97 mile walk to the Hospitality House at OKW.
From the Congress Park bus stop to the Downtown Disney Marketplace entrance is about .5 miles. If you're in (or close to) this section, it's a quick walk to the DTD Marketplace bus stop where you can pick up an OKW bus.
If you're not into walking, you can take the internal SSR bus to the DTD Marketplace and change buses to an OKW bus. (The DTD buses don't seem top run quite as often as the theme park buses. You could wait as much as 20-30 minutes at each bus stop if you hit the worst case scenerio.)
You can also do the same taking the boat. Grab the boat at the Carriage House, change at Pleasure Island, and catch the boat up to OKW. (This would probably take the longest amount of time.)
We found this out on our trip May 30-june 7.....unpleasant surprise when we went to get on at the Marketplace dock 
Took the bus that time.....but did take it later in the trip.
