diff --git a/scripts/check_oc_pqg_drift.py b/scripts/check_oc_pqg_drift.py
new file mode 100755
index 0000000..627fdb8
--- /dev/null
+++ b/scripts/check_oc_pqg_drift.py
@@ -0,0 +1,85 @@
+#!/usr/bin/env python3
+"""Check whether Eric Kansa's OC PQG files on GCS have drifted from our R2 mirror.
+
+Reads our latest.json + the per-file manifests from data.isamples.org/oc_pqg/,
+HEADs the GCS source, and reports whether upstream has a newer version.
+
+Exit codes:
+  0 — in sync, no drift
+  1 — drift detected (GCS has a different etag from what we've mirrored)
+  2 — probe failure (network error, malformed response, etc.)
+
+Run manually for now:
+    python scripts/check_oc_pqg_drift.py
+
+Later: wire to GitHub Actions cron.
+"""
+import json
+import sys
+import urllib.request
+
+LATEST_URL = "https://data.isamples.org/oc_pqg/latest.json"
+GCS_BASE = "https://storage.googleapis.com/opencontext-parquet/"
+GCS_FILES = {
+    "narrow": "oc_isamples_pqg.parquet",
+    "wide":   "oc_isamples_pqg_wide.parquet",
+}
+
+
+def fetch_json(url, timeout=20):
+    req = urllib.request.Request(url, headers={"User-Agent": "isamples-oc-drift-check/1.0"})
+    with urllib.request.urlopen(req, timeout=timeout) as r:
+        return json.loads(r.read())
+
+
+def head(url, timeout=20):
+    req = urllib.request.Request(url, method="HEAD",
+                                 headers={"User-Agent": "isamples-oc-drift-check/1.0"})
+    with urllib.request.urlopen(req, timeout=timeout) as r:
+        return dict(r.headers)
+
+
+def main() -> int:
+    try:
+        latest = fetch_json(LATEST_URL)
+    except Exception as e:
+        print(f"ERROR: could not fetch {LATEST_URL}: {e}", file=sys.stderr)
+        return 2
+
+    drift_any = False
+    for flavor, gcs_name in GCS_FILES.items():
+        flavor_ptr = latest.get(flavor)
+        if not flavor_ptr:
+            print(f"ERROR: latest.json has no entry for {flavor!r}", file=sys.stderr)
+            return 2
+
+        try:
+            manifest = fetch_json(f"https://data.isamples.org/{flavor_ptr['manifest']}")
+        except Exception as e:
+            print(f"ERROR: could not fetch manifest for {flavor}: {e}", file=sys.stderr)
+            return 2
+
+        try:
+            gcs_headers = head(f"{GCS_BASE}{gcs_name}")
+        except Exception as e:
+            print(f"ERROR: HEAD {GCS_BASE}{gcs_name}: {e}", file=sys.stderr)
+            return 2
+
+        gcs_etag = gcs_headers.get("ETag", "").strip('"')
+        gcs_last_modified = gcs_headers.get("Last-Modified", "")
+        our_etag = manifest.get("source_etag", "")
+        our_updated = manifest.get("source_updated", "")
+
+        in_sync = gcs_etag == our_etag
+        state = "in sync" if in_sync else "DRIFT"
+        print(f"[{flavor}] {state}")
+        print(f"    mirrored: etag={our_etag} updated={our_updated}")
+        print(f"    gcs:      etag={gcs_etag} last-modified={gcs_last_modified}")
+        if not in_sync:
+            drift_any = True
+
+    return 1 if drift_any else 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())
diff --git a/workers/data-isamples-org/src/index.js b/workers/data-isamples-org/src/index.js
index e1f5088..82e4e0d 100644
--- a/workers/data-isamples-org/src/index.js
+++ b/workers/data-isamples-org/src/index.js
@@ -18,7 +18,15 @@
  * working.
  */
 
-const IMMUTABLE_PATTERN = /^isamples_\d{6}_.*\.parquet$/;
+// Immutable-by-filename patterns. Match files whose path fully determines
+// their contents (filename includes a version / date stamp).
+//   - isamples_YYYYMM_*.parquet  (monthly iSamples snapshots)
+//   - oc_pqg/oc_isamples_pqg*_YYYYMMDD.parquet  (mirror of Eric Kansa's
+//     OpenContext PQG files — versioned by the upstream GCS updated-date)
+const IMMUTABLE_PATTERNS = [
+  /^isamples_\d{6}_.*\.parquet$/,
+  /^oc_pqg\/oc_isamples_pqg.*_\d{8}\.parquet$/,
+];
 const IMMUTABLE_MAX_AGE = 60 * 60 * 24 * 365; // 1 year
 const FALLBACK_MAX_AGE = 300; // 5 minutes
 
@@ -72,7 +80,7 @@ export default {
     for (const [k, v] of Object.entries(CORS_HEADERS)) headers.set(k, v);
 
     // Cache-Control: this is the optimization.
-    if (IMMUTABLE_PATTERN.test(key)) {
+    if (IMMUTABLE_PATTERNS.some(p => p.test(key))) {
       headers.set('Cache-Control', `public, max-age=${IMMUTABLE_MAX_AGE}, immutable`);
     } else {
       headers.set('Cache-Control', `public, max-age=${FALLBACK_MAX_AGE}`);