In an embodiment, an oxygen sensor comprises a giant magnetoresistance device (10), and a magnetic field generator (14, 14a, 14b) arranged to generate a magnetic field (12, 12a, 12b) overlapping the giant magnetoresistance device and an examination region (20). A component (Bx) of the magnetic field detected by the giant magnetoresistance device is dependent upon an oxygen concentration in the examination region. In an embodiment, a chip (40) includes one or more electrically conductive traces (14a, 14b) disposed on or in the chip and a giant magnetoresistance device (10) disposed on or in the chip such that electrical current flowing in the trace or traces generates a magnetic field (12a, 12b) that overlaps the magnetic field sensor, said magnetic field being perturbed (Bx) by ambient oxygen (24) such that a signal output by the magnetic field sensor indicates ambient oxygen concentration.