An oxygen sensor arrangement for sensing the oxygen in a breathing loop of a breathing apparatus, which oxygen sensor arrangement comprises: - at least one primary oxygen sensor (30, 31, 32) arranged to operatively measure the oxygen in the breathing loop, and - a control arrangement for obtaining measures from said oxygen sensor, and - a test channel arrangement (15, 362) adapted to operatively provide a first gas having a first known fraction of oxygen from a first gas supply to said primary oxygen sensor (30, 31, 32) at a position (267, 267', 267", 364, 364', 364") adjacent to or directly adjacent to said primary oxygen sensor, and - at least a first test valve arrangement (46, 355, 356, 357, 372) arranged to operatively open and close the flow of said first gas through said test channel arrangement (15, 362), and - said control arrangement being arranged to operatively actuate said first test valve arrangement (46, 355, 356, 357, 372) so as to provide an amount of said first gas to said primary oxygen sensor (30, 31, 32) via said test channel arrangement (15, 362), and - said control arrangement arranged to operatively obtain measures from said primary oxygen sensor (30, 31, 32), characterized in that - said oxygen sensor arrangement comprises a cavity (33, 34, 35) configured to be in fluid communication with the breathing loop (29, 29'), and - said at least one primary oxygen sensor (30, 31, 32) is arranged in the cavity (33, 34, 35), and - said cavity (33, 34, 35) is provided with at least one output orifice (263, 263', 263", 264, 264', 264", 267, 267', 267") for the test channel arrangement (15, 362) arranged at a position adjacent to or directly adjacent to said oxygen sensor (30, 31, 32) such that said first gas can be operatively injected at an oblique angle with respect to the sensing surface of the primary oxygens sensor (30, 31, 32). - at least one primary oxygen sensor (30, 31, 32) arranged to operatively measure the oxygen in the breathing loop, and - a control arrangement for obtaining measures from said oxygen sensor, and - a test channel arrangement (15, 362) adapted to operatively provide a first gas having a first known fraction of oxygen from a first gas supply to said primary oxygen sensor (30, 31, 32) at a position (267, 267', 267", 364, 364', 364") adjacent to or directly adjacent to said primary oxygen sensor, and - at least a first test valve arrangement (46, 355, 356, 357, 372) arranged to operatively open and close the flow of said first gas through said test channel arrangement (15, 362), and - said control arrangement being arranged to operatively actuate said first test valve arrangement (46, 355, 356, 357, 372) so as to provide an amount of said first gas to said primary oxygen sensor (30, 31, 32) via said test channel arrangement (15, 362), and - said control arrangement arranged to operatively obtain measures from said primary oxygen sensor (30, 31, 32), - said oxygen sensor arrangement comprises a cavity (33, 34, 35) configured to be in fluid communication with the breathing loop (29, 29'), and - said at least one primary oxygen sensor (30, 31, 32) is arranged in the cavity (33, 34, 35), and - said cavity (33, 34, 35) is provided with at least one output orifice (263, 263', 263", 264, 264', 264", 267, 267', 267") for the test channel arrangement (15, 362) arranged at a position adjacent to or directly adjacent to said oxygen sensor (30, 31, 32) such that said first gas can be operatively injected at an oblique angle with respect to the sensing surface of the primary oxygens sensor (30, 31, 32).