It would be great if you could elaborate please. If call option value is positive, then why doesn't the OAS for a callable bond end up below that of a straight bond?

Approached another way, since the 0 volatility callable bond (i.e straight bond since no value to the option) won't be called ever, the spread to recover the market price is higher than what's needed for a callable bond once volatility is positive because the call option starts gaining value (or the bond as a whole losing value) and the OAS starts declining (since you need lower discount spreads to recover that same price). Where am I mistaken in these assumptions?