Q: is it normal for an engine light that's been reset after service to come back on?

Any given diagnostic trouble code will typically have multiple faults listed as potential causes of "the" code setting. It is possible for one fault (in your case an oxygen sensor that might not be putting out the expected waveform) to be addressed in a repair and yet additional faults exist where, again, the "additional" faults are related to the original code that was downloaded. However, it is also possible that the check engine light (CEL) has come on for a new reason (a new fault) that is completely unrelated to the code that involved the oxygen sensor. There are literally thousands of diagnostic trouble codes on modern vehicles, representing countless vehicle functions, that could cause the check engine light to come on. The only way to determine whether the CEL is presently illuminated because of a new or the "old" code is to have the codes read again. To do that, please request a check engine light diagnostic and, in the context of that you could request that the original mechanic come back as he will of course be most familiar with all of the circumstances. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.