diff --git a/dev-python/cython/cython-3.1.7.ebuild b/dev-python/cython/cython-3.1.7.ebuild
index 50e541c8414b8..5b79d143300b8 100644
--- a/dev-python/cython/cython-3.1.7.ebuild
+++ b/dev-python/cython/cython-3.1.7.ebuild
@@ -69,6 +69,10 @@ python_test() {
 	# Needed to avoid confusing cache tests
 	unset CYTHON_FORCE_REGEN
 
+	# uses $(nproc) to additionally parallelize many OpenMP-based jobs,
+	# leading to overcommitting
+	local -x OMP_NUM_THREADS=1
+
 	tc-export CC
 
 	local testargs=(
diff --git a/dev-python/cython/cython-3.1.8.ebuild b/dev-python/cython/cython-3.1.8.ebuild
index 3874d1e721664..cea99bf41054a 100644
--- a/dev-python/cython/cython-3.1.8.ebuild
+++ b/dev-python/cython/cython-3.1.8.ebuild
@@ -69,6 +69,10 @@ python_test() {
 	# Needed to avoid confusing cache tests
 	unset CYTHON_FORCE_REGEN
 
+	# uses $(nproc) to additionally parallelize many OpenMP-based jobs,
+	# leading to overcommitting
+	local -x OMP_NUM_THREADS=1
+
 	tc-export CC
 
 	local testargs=(
diff --git a/dev-python/cython/cython-3.2.3.ebuild b/dev-python/cython/cython-3.2.3.ebuild
index 23fc59df0adf0..1d9399568f5b9 100644
--- a/dev-python/cython/cython-3.2.3.ebuild
+++ b/dev-python/cython/cython-3.2.3.ebuild
@@ -1,4 +1,4 @@
-# Copyright 1999-2025 Gentoo Authors
+# Copyright 1999-2026 Gentoo Authors
 # Distributed under the terms of the GNU General Public License v2
 
 EAPI=8
@@ -69,6 +69,10 @@ python_test() {
 	# Needed to avoid confusing cache tests
 	unset CYTHON_FORCE_REGEN
 
+	# uses $(nproc) to additionally parallelize many OpenMP-based jobs,
+	# leading to overcommitting
+	local -x OMP_NUM_THREADS=1
+
 	tc-export CC
 
 	local testargs=(
diff --git a/dev-python/cython/cython-3.2.4.ebuild b/dev-python/cython/cython-3.2.4.ebuild
index f4ac503033bd1..1d9399568f5b9 100644
--- a/dev-python/cython/cython-3.2.4.ebuild
+++ b/dev-python/cython/cython-3.2.4.ebuild
@@ -69,6 +69,10 @@ python_test() {
 	# Needed to avoid confusing cache tests
 	unset CYTHON_FORCE_REGEN
 
+	# uses $(nproc) to additionally parallelize many OpenMP-based jobs,
+	# leading to overcommitting
+	local -x OMP_NUM_THREADS=1
+
 	tc-export CC
 
 	local testargs=(