OAS and default risk concept, valuation, corporate, only, default

transferpricing

R29 Relative-Value Methodologies for Global Credit Bond Portfolio Management
P.76 Spread analysis

"Given the exclusion of default risk in OAS option-valuation models,
OAS valuation has seen only limited extention into the higher-risk markets of the
quasi-equity, high-yield corporate, and EMG-debt asset classes."

what does this sentence mean?

OAS doesn't reflect default risk??

Please explain this concept...