Help with codes

1997 Cadillac Seville sls
Came up with the following engine codes (Check engine light on)

# P1139
# P0138

I had a check engine light come on a few months ago and the mecahnic who worked on the car said it was a bad oxygen sensor. I had it replaced, and within a month or so, the check engine light came on again. Brought it back to same mechanic who said it was the oxygen sensor again, and that he would replace it NO CHARGE. Now once again, a month later, the check engine light is on again.

The codes that came up that I checked at AUTO ZONE are listed above.
Does anyone know what those codes mean, and has anyone else had similar issue??? What did you do to correct it?

THANKS
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If it has tits or tires, it needs to be serviced! :alchi: