Relative OAS Valuation

I’m having trouble clearly understanding what figure 3 on page 87 in book 5 is stating: Treasury-Benchmark Sector-B.M. Issuer-Specific-B.M. OAS = 0 Overvalued Overvalued Fairly Priced OAS < 0 Overvalued Overvalued Overvalued The first part makes sense (didn’t include in above), if the actual OAS < required OAS then the bond is over-valued because it doesn’t give you enough in the spread. But what exactly do they mean =0? Does that mean if the Option-Adjust Spread = i.e. Treasury spread, it is overvalued? Shouldn’t the Option-Adjust Spread be less because it doesn’t factor the option cost?