  Mixed-morphology supernova remnants (MM SNRs) are a mysterious class of
objects that display thermal X-ray emission within their radio shell. They are
an older class of SNRs, and as such are profoundly affected by the environment
into which they evolve. VRO 42.05.01 is a MM SNR of puzzling morphology in the
direction of the Galactic anticentre. Low-frequency radio observations of
supernova remnants are sensitive to synchrotron electrons accelerated in the
shock front. We aim to compare the low-frequency emission to higher frequency
observations to understand the environmental and shock acceleration conditions
that have given rise to the observed properties of this source. We present a
LOFAR High Band Antenna map centred at 143 MHz of the region of the Galactic
plane centred at $l = 166^\rm{o}, b = 3.5^\rm{o}$ at 143 MHz, with a resolution
of 148'' and an rms noise of 4.4 mJy bm$^{-1}$ . Our map is sensitive to scales
as large as $6^\rm{o}$. We compared the LOw Frequency ARay (LOFAR) observations
to archival higher frequency radio, infrared, and optical data to study the
emission properties of the source in different spectral regimes. We did this
both for the SNR and for OA 184, an H II region within our field of view. We
find that the radio spectral index of VRO 42.05.01 increases at low radio
frequencies; i.e. the LOFAR flux is higher than expected from the measured
spectral index value at higher radio frequencies. This observed curvature in
the low-frequency end of the radio spectrum occurs primarily in the brightest
regions of the source, while the fainter regions present a roughly constant
power-law behaviour between 143 MHz and 2695 MHz. We favour an explanation for
this steepening whereby radiative shocks have high compression ratios and
electrons of different energies probe different length scales across the
shocks, therefore sampling regions of different compression ratios.
