Sarutobi ain't that weak. It depends, is Saru in his prime??? Or the old version...
In his prime : Probably, its definitely a very high chance even if Onoki is young( i cant imagine a short young man?) but depends on the circumstances i think
Old version: with additional wrinkles, i dont think saru stands a chance.
Onoki has a very risky body for any battle it can give out on him at any time, Sarutobi never had that problem although old age was catching up with him he was still able to fight without any real physical problems other than having less stamina.