diff --git a/src/medunda/plotter.py b/src/medunda/plotter.py
index 8079923..55c6250 100644
--- a/src/medunda/plotter.py
+++ b/src/medunda/plotter.py
@@ -13,7 +13,7 @@
 LOGGER = logging.getLogger(__name__)
 
 VAR_METADATA = {
-    "o2": {"label": "Oxygen", "unit": "µmol/m³", "cmap": cmocean.cm.deep},  # pyright: ignore[reportAttributeAccessIssue]
+    "o2": {"label": "Oxygen", "unit": "mmol/m³", "cmap": cmocean.cm.deep},  # pyright: ignore[reportAttributeAccessIssue]
     "chl": {
         "label": "Chlorophyll-a",
         "unit": "mg/m³",
diff --git a/src/medunda/tools/units.py b/src/medunda/tools/units.py
new file mode 100644
index 0000000..6450d9a
--- /dev/null
+++ b/src/medunda/tools/units.py
@@ -0,0 +1,41 @@
+import logging
+
+from medunda.components.dataset import Dataset
+from medunda.plotter import VAR_METADATA
+
+logger = logging.getLogger(__name__)
+
+
+def check_variable_units(var):
+    """
+    Check if a single variable has a "unit" attribute.
+    """
+    var_name = var.name
+    units = var.attrs.get("units")
+    result = {
+        "variable": var_name,
+        "units": units,
+    }
+    return result
+
+
+def check_units(ds: Dataset, fill_from_metadata: bool = True):
+    for var_name in ds.data_vars:
+        var = ds[var_name]
+        units = ds.attrs.get("units", None)
+
+        if units is None and fill_from_metadata:
+            unit_meta = VAR_METADATA.get(var_name, {}).get("unit")
+            if unit_meta:
+                var.attrs["units"] = unit_meta
+                logger.warning(
+                    f"Variable {var_name} have no unit so it is filled from VAR_METADATA"
+                )
+            else:
+                raise ValueError(f"Variable {var_name} has no units")
+
+
+def check_file_units():
+    """Open a NetCDF file and check the presence of a variable.
+    If a unit is missing, it fills it from VAR_METADATA."""
+    pass