  We present velocity resolved SOFIA/upGREAT observations of [OI] and [CII]
lines toward a Class I protostar, L1551 IRS 5, and its outflows. The SOFIA
observations detect [OI] emission toward only the protostar and [CII] emission
toward the protostar and the red-shifted outflow. The [OI] emission has a width
of $\sim$100 km s$^{-1}$ only in the blue-shifted velocity, suggesting an
origin in shocked gas. The [CII] lines are narrow, consistent with an origin in
a photodissociation region. Differential dust extinction from the envelope due
to the inclination of the outflows is the most likely cause of the missing
red-shifted [OI] emission. Fitting the [OI] line profile with two Gaussian
components, we find one component at the source velocity with a width of
$\sim$20 km s$^{-1}$ and another extremely broad component at -30 km s$^{-1}$
with a width of 87.5 km s$^{-1}$, the latter of which has not been seen in
L1551 IRS 5. The kinematics of these two components resemble cavity shocks in
molecular outflows and spot shocks in jets. Radiative transfer calculations of
the [OI], high-J CO, and H$_2$O lines in the cavity shocks indicate that [OI]
dominates the oxygen budget, making up more than 70% of the total gaseous
oxygen abundance and suggesting [O]/[H] of $\sim$1.5$\times$10$^{-4}$.
Attributing the extremely broad [OI] component to atomic winds, we estimate the
intrinsic mass loss rate of (1.3$\pm$0.8)$\times$ 10$^{-6}$ M$_{\odot}$
yr$^{-1}$. The intrinsic mass loss rates derived from low-J CO, [OI], and HI
are similar, supporting the model of momentum-conserving outflows, where the
atomic wind carries most momentum and drives the molecular outflows.

Astrophysics of Galaxies;Solar and Stellar Astrophysics