  We report on sensitive far-infrared observations of $^{12}$CO pure rotational
transitions in the OMC-1 core of Orion. The lines were observed with the Long
Wavelength Spectrometer (LWS) in the grating mode on board the Infrared Space
Observatory (ISO), covering the 43-197 $\mu$m wavelength range. The transitions
from $J_{up}=14$ up to $J_{up}=19$ have been identified across the whole OMC-1
core and lines up to $J_{up}= 43$ have been detected towards the central
region, KL/IRc2. In addition, we have taken high-quality spectra in the
Fabry-Perot mode of some of the CO lines. In KL/IRc2 the lines are
satisfactorily accounted for by a three-temperature model describing the
plateau and ridge emission. The fluxes detected in the high-$J$ transitions
($J_{up} > 34$) reveal the presence of a very hot and dense gas component
($T=1500-2500$ K; $\rm N(CO)$=2\times 10^{17}\cmmd$), probably originating from
some of the embedded sources previously observed in the $\rm H_2$ near-infrared
lines. At all other positions in the OMC-1 core, we estimate kinetic
temperatures $\geq 80$ K and as high as 150 K at some positions around IRc2,
from a simple Large-Velocity Gradient model.
