diff --git a/gnommo/cli.py b/gnommo/cli.py
index 77da238..d4aaa58 100644
--- a/gnommo/cli.py
+++ b/gnommo/cli.py
@@ -4279,6 +4279,146 @@ def _render_log(msg: str) -> None:
             pass
 
 
+def _system_mem():
+    """Return (available_bytes, total_bytes) of physical RAM, or None.
+
+    Cross-platform without requiring psutil (the machine that OOM-crashes is
+    Windows): prefers psutil, falls back to Windows GlobalMemoryStatusEx and
+    Linux /proc/meminfo so system-memory pressure is visible everywhere.
+    """
+    try:
+        import psutil
+
+        vm = psutil.virtual_memory()
+        return vm.available, vm.total
+    except Exception:
+        pass
+    if sys.platform.startswith("win"):
+        try:
+            import ctypes
+
+            class _MEMSTAT(ctypes.Structure):
+                _fields_ = [
+                    ("dwLength", ctypes.c_ulong),
+                    ("dwMemoryLoad", ctypes.c_ulong),
+                    ("ullTotalPhys", ctypes.c_ulonglong),
+                    ("ullAvailPhys", ctypes.c_ulonglong),
+                    ("ullTotalPageFile", ctypes.c_ulonglong),
+                    ("ullAvailPageFile", ctypes.c_ulonglong),
+                    ("ullTotalVirtual", ctypes.c_ulonglong),
+                    ("ullAvailVirtual", ctypes.c_ulonglong),
+                    ("ullAvailExtendedVirtual", ctypes.c_ulonglong),
+                ]
+
+            m = _MEMSTAT()
+            m.dwLength = ctypes.sizeof(_MEMSTAT)
+            ctypes.windll.kernel32.GlobalMemoryStatusEx(ctypes.byref(m))
+            return int(m.ullAvailPhys), int(m.ullTotalPhys)
+        except Exception:
+            return None
+    if sys.platform.startswith("linux"):
+        try:
+            info = {}
+            with open("/proc/meminfo") as f:
+                for line in f:
+                    k, _, v = line.partition(":")
+                    info[k.strip()] = int(v.strip().split()[0]) * 1024
+            return info.get("MemAvailable", info.get("MemFree", 0)), info.get("MemTotal", 0)
+        except Exception:
+            return None
+    if sys.platform == "darwin":
+        try:
+            import os as _os
+
+            page = _os.sysconf("SC_PAGE_SIZE")
+            total = _os.sysconf("SC_PHYS_PAGES") * page
+            out = subprocess.run(["vm_stat"], capture_output=True, text=True).stdout
+            free = inactive = spec = 0
+            for line in out.splitlines():
+                num = line.split(":")[-1].strip().rstrip(".")
+                if line.startswith("Pages free:"):
+                    free = int(num)
+                elif line.startswith("Pages inactive:"):
+                    inactive = int(num)
+                elif line.startswith("Pages speculative:"):
+                    spec = int(num)
+            return (free + inactive + spec) * page, total
+        except Exception:
+            return None
+    return None
+
+
+def _start_memory_sampler(logfile, interval: float = 3.0):
+    """Log memory every `interval`s in a background thread → OOM visibility.
+
+    Records system free/used RAM (always) and the gnommo+ffmpeg process-tree RSS
+    (when psutil is available). Returns a stop callback + peak dict, or None. On a
+    hard OOM kill the finally block never runs, but the per-sample lines are
+    flushed as they happen, so the log shows memory climbing right up to the kill.
+    """
+    import threading
+
+    total = _system_mem()
+    if total is None:
+        _render_log("memory sampler: unavailable on this platform")
+        return None
+
+    try:
+        import psutil
+
+        _proc = psutil.Process()
+    except Exception:
+        psutil = None
+        _proc = None
+        _render_log(
+            "memory sampler: system RAM only ('pip install psutil' adds per-process RSS)"
+        )
+
+    stop = threading.Event()
+    peak = {"rss": 0, "used_pct": 0.0}
+
+    def _tree_rss():
+        if _proc is None:
+            return None
+        try:
+            rss = _proc.memory_info().rss
+            for c in _proc.children(recursive=True):
+                try:
+                    rss += c.memory_info().rss
+                except Exception:
+                    pass
+            return rss
+        except Exception:
+            return None
+
+    def _loop():
+        while not stop.wait(interval):
+            mem = _system_mem()
+            if not mem:
+                continue
+            avail, tot = mem
+            used_pct = 100.0 * (tot - avail) / tot if tot else 0.0
+            peak["used_pct"] = max(peak["used_pct"], used_pct)
+            rss = _tree_rss()
+            if rss is not None:
+                peak["rss"] = max(peak["rss"], rss)
+                rss_str = f"gnommo+ffmpeg={rss / 1e9:.2f}GB | "
+            else:
+                rss_str = ""
+            try:
+                logfile.write(
+                    f"[mem {datetime.now().strftime('%H:%M:%S')}] {rss_str}"
+                    f"system {used_pct:.0f}% used, {avail / 1e9:.2f}GB free of {tot / 1e9:.1f}GB\n"
+                )
+                logfile.flush()
+            except Exception:
+                pass
+
+    t = threading.Thread(target=_loop, daemon=True)
+    t.start()
+    return stop, peak
+
+
 def _write_render_log_header(logfile, project_path, res, slides_arg, force, chunk_slides):
     import os
     import platform as _platform
@@ -4289,13 +4429,12 @@ def _write_render_log_header(logfile, project_path, res, slides_arg, force, chun
     except Exception:
         ffmpeg_ver = "unavailable"
 
-    try:
-        import psutil  # optional dependency
-
-        vm = psutil.virtual_memory()
-        mem = f"{vm.available / 1e9:.1f} GB free / {vm.total / 1e9:.1f} GB total"
-    except Exception:
-        mem = "unknown (install psutil for memory info)"
+    _m = _system_mem()
+    mem = (
+        f"{_m[0] / 1e9:.1f} GB free / {_m[1] / 1e9:.1f} GB total"
+        if _m
+        else "unknown"
+    )
 
     logfile.write("=" * 70 + "\n")
     logfile.write(f"gnommo render log — {project_path.name}\n")
@@ -4355,6 +4494,7 @@ def cmd_render(
     sys.stdout = _TeeStream(_orig_out, logfile)
     sys.stderr = _TeeStream(_orig_err, logfile)
     _RENDER_LOGFILE = logfile
+    _sampler = _start_memory_sampler(logfile)
     try:
         return _cmd_render_impl(project_path, verbose, dry_run, **passthrough)
     except BaseException:
@@ -4365,6 +4505,13 @@ def cmd_render(
         logfile.flush()
         raise
     finally:
+        if _sampler is not None:
+            _stop, _peak = _sampler
+            _stop.set()
+            _peak_rss = f"{_peak['rss'] / 1e9:.2f} GB tree RSS, " if _peak["rss"] else ""
+            logfile.write(
+                f"\n[peak memory] {_peak_rss}system peaked at {_peak['used_pct']:.0f}% used\n"
+            )
         sys.stdout = _orig_out
         sys.stderr = _orig_err
         _RENDER_LOGFILE = None
@@ -4710,18 +4857,16 @@ def _cmd_render_impl(
         output_path = _output_path_override
         out_dir = output_path.parent
         out_filename = output_path.name
-    elif config.output_video:
-        out_filename = config.output_video
-        out_dir = project_path / "out" / res if res != "full" else project_path / "out"
-        output_path = out_dir / out_filename
-    elif slide_range:
-        start, end = slide_range
-        range_suffix = f"_{start}-{end}" if end else f"_{start}-end"
-        out_filename = f"final{range_suffix}.mp4"
-        out_dir = project_path / "out" / res if res != "full" else project_path / "out"
-        output_path = out_dir / out_filename
     else:
-        out_filename = f"{config.co}.mp4"
+        base = config.output_video if config.output_video else f"{config.co}.mp4"
+        # A partial (--slides) render appends the range to the filename so that
+        # e.g. S1:S9 and S10:S19 don't overwrite each other (or the full render).
+        if slide_range:
+            start, end = slide_range
+            rng = f"{start}_{end}" if end else f"{start}_end"
+            base_p = Path(base)
+            base = f"{base_p.stem}_{rng}{base_p.suffix or '.mp4'}"
+        out_filename = base
         out_dir = project_path / "out" / res if res != "full" else project_path / "out"
         output_path = out_dir / out_filename