diff --git a/build/build-jasp.sh b/build/build-jasp.sh
index 91ab7aa..7b41022 100755
--- a/build/build-jasp.sh
+++ b/build/build-jasp.sh
@@ -260,6 +260,15 @@ PY
 # Retrying is the cheap fix: renv skips what is already installed, so each pass
 # starts from more complete state and converges. Passes are capped so a genuine
 # failure cannot loop forever.
+# OpenMx instantiates enough Eigen templates that a single cc1plus can take
+# several GB. At the top-level -j the compiler gets OOM-killed
+# ("g++: fatal error: Killed signal terminated program cc1plus"), which is not
+# a race — it stalls at the same module count on every retry. Compile R packages
+# with less parallelism than the C++ build uses.
+R_JOBS="${R_JOBS:-2}"
+export MAKEFLAGS="-j${R_JOBS}"
+log "compiling R packages with -j${R_JOBS} (OpenMx needs the headroom)"
+
 EXPECTED_MODULES=22
 for attempt in 1 2 3; do
   log "module build pass ${attempt}/3"