diff --git a/src/blosc2/blosc2_ext.pyx b/src/blosc2/blosc2_ext.pyx
index de90e254c..dd0e3af8f 100644
--- a/src/blosc2/blosc2_ext.pyx
+++ b/src/blosc2/blosc2_ext.pyx
@@ -6861,6 +6861,30 @@ cdef class NDArray:
         _check_rc(rc, "Error while getting the sparse selection")
         return arr
 
+    cdef _check_oindex_buffer(self, arr, key, str role):
+        """Raise ValueError if the buffer of `arr` (the `role` "destination" or "source" in the
+        message) cannot hold the orthogonal selection `key`.
+
+        The C copy moves typesize bytes per item, but an array's dtype can have smaller items:
+        arrays that c-blosc2 2.7-2.16 wrote without a dtype store "|u1" whatever the typesize,
+        or the dtype string is damaged.  A buffer allocated for the dtype is too small then.
+        """
+        cdef int64_t typesize = self.array.sc.typesize
+        nbytes = typesize  # a Python int: it cannot wrap around for huge selections
+        for k in key:
+            nbytes *= len(k)
+        cdef Py_buffer view
+        PyObject_GetBuffer(arr, &view, PyBUF_SIMPLE)
+        cdef Py_ssize_t buflen = view.len
+        PyBuffer_Release(&view)
+        if buflen < nbytes:
+            msg = (f"{role} buffer is smaller than the requested orthogonal selection "
+                   f"({buflen} < {nbytes} bytes)")
+            itemsize = getattr(arr, "itemsize", typesize)
+            if itemsize != typesize:
+                msg += f": its itemsize is {itemsize}, but the array's typesize is {typesize}"
+            raise ValueError(msg)
+
     @_handle_locked
     def get_oindex_numpy(self, arr, key):
         """
@@ -6868,6 +6892,7 @@ cdef class NDArray:
         """
         if len(key) != self.array.ndim:
             raise ValueError(f"Key must have {self.array.ndim} dimensions, got {len(key)}.")
+        self._check_oindex_buffer(arr, key, "destination")
         cdef int64_t[B2ND_MAX_DIM] buffershape_
         cdef int64_t** key_
         cdef int64_t buffersize_ = self.array.sc.typesize
@@ -6901,6 +6926,7 @@ cdef class NDArray:
         """
         if len(key) != self.array.ndim:
             raise ValueError(f"Key must have {self.array.ndim} dimensions, got {len(key)}.")
+        self._check_oindex_buffer(arr, key, "source")
         cdef int64_t[B2ND_MAX_DIM] buffershape_
         cdef int64_t** key_
         cdef int64_t buffersize_ = self.array.sc.typesize
diff --git a/tests/data/legacy_u1_dtype/example_day_month_temp.b2nd b/tests/data/legacy_u1_dtype/example_day_month_temp.b2nd
new file mode 100644
index 000000000..3f2a09c24
Binary files /dev/null and b/tests/data/legacy_u1_dtype/example_day_month_temp.b2nd differ
diff --git a/tests/ndarray/test_oindex_typesize.py b/tests/ndarray/test_oindex_typesize.py
new file mode 100644
index 000000000..010c37816
--- /dev/null
+++ b/tests/ndarray/test_oindex_typesize.py
@@ -0,0 +1,120 @@
+#######################################################################
+# Copyright (c) 2019-present, Blosc Development Team <blosc@blosc.org>
+# All rights reserved.
+#
+# SPDX-License-Identifier: BSD-3-Clause
+#######################################################################
+
+# Orthogonal selections copy typesize bytes per item in C into a NumPy buffer allocated for
+# the array's dtype.  Arrays whose dtype has smaller items than the typesize must raise
+# instead of overflowing that buffer.
+
+import shutil
+from pathlib import Path
+
+import numpy as np
+import pytest
+
+import blosc2
+
+# Written by c-blosc2's examples/b2nd/example_frame_generator.c (c-blosc2 2.7-2.16 store
+# "|u1" in the b2nd metalayer of arrays created without a dtype): float32 items, typesize 4
+LEGACY_U1_FILE = Path(__file__).parent.parent / "data" / "legacy_u1_dtype" / "example_day_month_temp.b2nd"
+
+
+def with_dtype_string(arr, old, new):
+    """`arr` with dtype string `old` replaced by `new` (same length) in its b2nd metalayer"""
+    assert len(old) == len(new)
+    cframe = arr.to_cframe()
+    # The metalayers come before the chunks, so the first match is the dtype
+    return blosc2.ndarray_from_cframe(cframe.replace(old.encode(), new.encode(), 1), copy=True)
+
+
+def assert_oindex_raises(arr):
+    """The paths to the C orthogonal selection raise ValueError and leave the array unchanged"""
+    before = arr.tobytes()
+    full = [list(range(n)) for n in arr.shape]
+    match = f"typesize is {arr.schunk.typesize}"
+    with pytest.raises(ValueError, match=match):
+        arr.get_oselection_numpy(full)
+    with pytest.raises(ValueError, match=match):
+        arr.get_oselection_numpy([[0]] * arr.ndim)  # one item: a few bytes too many
+    with pytest.raises(ValueError, match=match):
+        arr.oindex[tuple(full)] = np.zeros(arr.shape, dtype=arr.dtype)
+    if arr.ndim > 1:
+        with pytest.raises(ValueError, match=match):
+            arr[[0, 1]]  # through take(axis=0)
+    assert arr.tobytes() == before
+
+
+def test_legacy_u1_file(tmp_path):
+    path = tmp_path / LEGACY_U1_FILE.name
+    shutil.copy(LEGACY_U1_FILE, path)
+    arr = blosc2.open(path, mode="a")
+    assert arr.dtype == np.uint8
+    assert arr.schunk.typesize == 4
+    assert arr.shape == (400, 3)
+    assert_oindex_raises(arr)
+    assert path.read_bytes() == LEGACY_U1_FILE.read_bytes()
+
+
+@pytest.mark.parametrize(
+    ("dtype", "shape", "chunks", "blocks", "stored"),
+    [
+        # As c-blosc2's plugins/test_data/example_item_prices.b2nd
+        (np.float32, (12, 25, 250), (8, 10, 50), (4, 5, 10), "|u1"),
+        (np.float64, (100,), (30,), (7,), "|u1"),
+        # A damaged dtype string
+        (np.float64, (6, 10), (4, 4), (2, 2), "<f2"),
+    ],
+)
+def test_dtype_smaller_than_typesize(dtype, shape, chunks, blocks, stored):
+    data = np.arange(np.prod(shape), dtype=dtype).reshape(shape)
+    arr = with_dtype_string(blosc2.asarray(data, chunks=chunks, blocks=blocks), data.dtype.str, stored)
+    assert arr.dtype == np.dtype(stored)
+    assert arr.schunk.typesize == data.itemsize
+    assert_oindex_raises(arr)
+
+
+def test_oindex_buffer_size():
+    shape = (6, 10)
+    data = np.arange(np.prod(shape), dtype=np.int32).reshape(shape)
+    arr = blosc2.asarray(data, chunks=(4, 4), blocks=(2, 2))
+    key = [[5, 0, 2], [9, 1]]
+    expected = data[np.ix_(*key)]
+    out = np.empty(expected.shape, dtype=arr.dtype)
+    np.testing.assert_array_equal(arr.get_oindex_numpy(out, key), expected)
+    with pytest.raises(ValueError, match="destination buffer is smaller"):
+        arr.get_oindex_numpy(np.empty(expected.size - 1, dtype=arr.dtype), key)
+    with pytest.raises(ValueError, match="source buffer is smaller"):
+        arr.set_oindex_numpy(key, np.empty(expected.size - 1, dtype=arr.dtype))
+    np.testing.assert_array_equal(arr[...], data)
+    # A larger buffer is fine, as for slices
+    out = np.zeros(expected.size + 3, dtype=arr.dtype)
+    arr.get_oindex_numpy(out, key)
+    np.testing.assert_array_equal(out[: expected.size].reshape(expected.shape), expected)
+    # Empty selections need no buffer
+    arr.get_oindex_numpy(np.empty(0, dtype=arr.dtype), [[], [1]])
+    arr.set_oindex_numpy([[2], []], np.empty(0, dtype=arr.dtype))
+    np.testing.assert_array_equal(arr[...], data)
+
+
+@pytest.mark.parametrize(
+    "dtype",
+    ["?", ">i4", "S7", "<U100", [("a", "<i4"), ("b", "<f8")]],
+)
+def test_oindex_valid_dtypes(dtype):
+    # The typesize of valid arrays is the dtype's itemsize, also above 255 bytes ("<U100")
+    dtype = np.dtype(dtype)
+    shape = (5, 7)
+    data = np.arange(np.prod(shape)).astype(dtype).reshape(shape)
+    arr = blosc2.asarray(data, chunks=(3, 4), blocks=(2, 2))
+    assert arr.schunk.typesize == dtype.itemsize
+    key = [[4, 0, 2], [6, 1, 1]]
+    np.testing.assert_array_equal(arr.oindex[tuple(key)], data[np.ix_(*key)])
+    key = [[4, 0, 2], [6, 1, 3]]
+    value = data[np.ix_([0, 1, 2], [0, 1, 2])]
+    arr.oindex[tuple(key)] = value
+    expected = data.copy()
+    expected[np.ix_(*key)] = value
+    np.testing.assert_array_equal(arr[...], expected)