I did this last year using Wyeast 1007 German Ale. It worked out really well, and I'm planning to use it again this year. The major drawback is the beer won't clarify like a good oktoberfest should, but this yeast is a really nice alternative if you don't have lagering capabilities. It's a clean fermenting yeast that works really well for this style.
Kolsch ale yeast in an Oktoberfest recipe comes out really well. If you'd like good clarity, buy some whirlfloc tablets and use one of those per five gallons. Drop it in 5 minutes before flameout. I get really clear beers just using whirlfloc and not even cold crashing.