WEll, the codes will clear themselves after so many warm-up cycles. I believe it's about 40, so it sounds like that is what happened.
Now about the codes P0137 and P0157.
P0137 refers to HO2S-12 (Bank 1 Sensor 2) Circuit Low Input.
P0157 refers to HO2S-22 (Bank 2 Sensor 2) Circuit Low Input.
If you want more info on those codes, click here. The codes are listed twice, once with RPO code OSG and one without. From what I gather RPO code OSG stands for the Plant Code, Oshawa, Ontario, GM of Canada.
If the truck is running fine now, I would just ignore it. If they come back you have the diagnostic procedures and some possible causes in that doc.
You could have had some water in the gas. That is about the only thing I see from the possible causes that may have cured itself.