Option adjusted Spread?
Measure is used when a bond has an embedded option. Callable bond must have a greter yield than identical option free bond, and a greater nominal spread or z-spread. For embedded callable bonds - OAS<Z-spread. In other words you require more yield on the callable bond than for an option free bond.
For put option, you must pay for option meaning higher price, and OAS>Z-spread. Require less yield on the putable bond.
Easy way to remember- Zspread-OAS=Option cost in percent.
If put option you pay for option, Option Cost<0, meaning OAS is greater.
If call Option, you get paid meaning positive option cost, and greater z-spread.