  The chemistry of planet-forming disks sets the exoplanet atmosphere
composition and the prebiotic molecular content. Dust traps are of particular
importance as pebble growth and transport are crucial for setting the chemistry
where giant planets are forming. The asymmetric Oph IRS 48 dust trap located at
60 au radius provides a unique laboratory for studying chemistry in
pebble-concentrated environments in warm Herbig disks with low gas-to-dust
ratios down to 0.01. We use deep ALMA Band 7 line observations to search the
IRS 48 disk for H$_2$CO and CH$_3$OH line emission, the first steps of complex
organic chemistry. We report the detection of 7 H$_2$CO and 6 CH$_3$OH lines
with energy levels between 17 and 260 K. The line emission shows a crescent
morphology, similar to the dust continuum, suggesting that the icy pebbles play
an important role in the delivery of these molecules. Rotational diagrams and
line ratios indicate that both molecules originate from warm molecular regions
in the disk with temperatures $>$100 K and column densities $\sim10^{14}$
cm$^{-2}$ or a fractional abundance of $\sim10^{-8}$. Based on arguments from a
physical-chemical model with low gas-to-dust ratios, we propose a scenario
where the dust trap provides a huge icy grain reservoir in the disk midplane or
an `ice trap', which can result in high gas-phase abundances of warm COMs
through efficient vertical mixing. This is the first time that complex organic
molecules have been clearly linked to the presence of a dust trap. These
results demonstrate the importance of including dust evolution and vertical
transport in chemical disk models, as icy dust concentrations provide important
reservoirs for complex organic chemistry in disks.
