What is a common noun for the word occupation?

There isn't a single perfect common noun for "occupation," as it depends on the context. Here are some possibilities, with examples:

General:

* Job: "What is your job?"

* Work: "I'm looking for work."

* Profession: "She's a doctor by profession."

* Career: "He's trying to advance his career."

More specific:

* Trade: "He learned the trade of carpentry from his father."

* Field: "She's in the medical field."

* Role: "He's filling the role of project manager."

* Position: "The company is hiring for several open positions."

The best common noun to use will depend on what you're trying to communicate. For example, "job" is a more general term, while "profession" implies a higher level of skill and training.