Mi marido es muy observador y siempre se da cuenta cuando me corté el cabello.My husband is very observant and always notices when I've had my hair cut.
This means that the noun can be masculine or feminine, depending on the gender of person it refers to (e.g. el doctor, la doctora).
Los observadores informaron que no hubo irregularidades durante los comicios.The observers reported that there weren't any irregularities during the voting.