At the risk of....
At the risk of sounding slightly obscene, I say to you: the longer, the better. A regular length shaft (15"), just is not going to be long enough. With the "standard" Oday motor mounting location, a regular length shaft probably won't have its prop submerged. I agree with Steve, an extra long shaft would be better, although I bet a 20" would be adequate. I have a 20" on my Oday 23 (which I believe has the same stern and transom as the 25), and it is sufficiently deep to keep the prop in the water almost all of the time. I do wish I had the 25" shaft when it gets choppy.