It can either be fairly valued or over valued depending on the benchmark used.
If the OAS > OAS for benchmark > 0, it is undervalued - Long on this bond.
It the OAS
Higher the value of OAS, higher will be the value in the denominator for calculating the price of the bond, as a result lower will be the value, so, if OAS for a bond is more than the OAS for the benchmark, it means the value of the bond is lesser than the value of the benchmark and it is undervalued and is expected to rise in future. So long this security.
We can have a similar deduction for negative or
However, this is how I understand the concept. Comments and suggestions are welcomed.