We present SOFIA/FIFI-LS observations of five prototypical, low-mass Class I outflows (HH111, SVS13, HH26, HH34, HH30) in the far-infrared [OI]63um and [OI]145um transitions. Spectroscopic [OI]63um,145um maps enable us to study the spatial extent of warm, low-excitation atomic gas within outflows driven by Class I protostars. These [OI] maps may potentially allow us to measure the mass-loss rates (dM/dt_jet_) of this warm component of the atomic jet. A fundamental tracer of warm (i.e. T~500-1500K), low-excitation atomic gas is the [OI]63um emission line, which is predicted to be the main coolant of dense dissociative J-type shocks caused by decelerated wind or jet shocks associated with protostellar outflows. Under these conditions, the [O I]63um line can be directly connected to the instantaneous mass ejection rate.Thus, by utilising spectroscopic [OI]63um maps, we wish to determine the atomic mass flux rate dM/dt_jet_ ejected from our target outflows. Strong [OI]63um emission is detected at the driving sources HH111IRS, HH34IRS, SVS13, as well as at the bow shock region, HH7. The detection of the [OI]63um line at HH26A and HH8/HH10 can be attributed to jet deflection regions. The far-infrared counterpart of the optical jet is detected in [OI]63um only for HH111, but not for HH34. We interpret the [OI]63um emission at HH111IRS, HH34IRS, and SVS13 to be coming primarily from a decelerated wind shock, whereas multiple internal shocks within the HH111 jet may cause most of the [O I]63um emission seen there. At HH30, no [O I]63um,145um was detected. The [OI]145um line detection is at noise level almost everywhere in our obtained maps. The observed outflow rates of our Class I sample are to the order of dM/dt_jet_~10^-6^M_{sun}_/yr, if proper shock conditions prevail. Independent calculations connecting the [OI]63um line luminosity and observable jet parameters with the mass-loss rate are consistent with the applied shock model and lead to similar mass-loss rates. Wediscuss applicability and caveats of both methods. High-quality spectroscopic [OI]63um maps of protostellar outflows at the jet driving source potentially allow a clear determination of the mass ejection rate.