I believe a higher OAS would imply that it is more undervalued compared to the benchmark bond…and a smaller OAS or even negative OAS would imply that the bond is overvalued. Somebody correct me if I’m wrong.
It’s important to distinguish the “relative to what”. If we are dealing with an equivalent bond, the one with the higher OAS will be cheaper because you are discounting cash flows at a higher rate (higher OAS). When you do that, the price of the bond is “cheaper”, thus a better buy.
Bond 1: 100/1.04 = 96.15
Bond 2: 100/1.0425 = 95.92 (OAS higher by 0.0025)
Assuming Bonds 1 and 2 can be considered equivalent, I’d rather buy Bond 2 and pocket that oh so sweet 23 cents.