Is a wife, who feels like a "servant" to her husband's mother, and who works while resentful and tired, and whose husband is an expatriate and she is not always in her own apartment, considered an oppressor or oppressed?

Serving the husband's mother is not obligatory upon the wife; rather, it is a favor and an act of kindness. Your feeling of being compelled to serve her is not an injustice to the husband or his mother. We advise you to be patient and kind to her, seeking reward from Allah.