diff --git a/Cargo.lock b/Cargo.lock
index 159b3b020d4..f7c9dcaa4f8 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -7026,9 +7026,9 @@ checksum = "384b8ab6d37215f3c5301a95a4accb5d64aa607f1fcb26a11b5303878451b4fe"
 
 [[package]]
 name = "onpair"
-version = "0.2.0"
+version = "0.2.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e546e7cd983c9998a5a52e9c7eecaf38e518f2e6fc9c9ec649e8b53748a96bb0"
+checksum = "34272d1ea63266148a9a5b2b96db80a45245d034862302735f2f93413a6f9172"
 dependencies = [
  "hashbrown 0.16.1",
  "rand 0.9.5",
diff --git a/Cargo.toml b/Cargo.toml
index 3c0368e190e..7a7c94aee79 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -203,7 +203,7 @@ num_enum = { version = "0.7.3", default-features = false }
 object_store = { version = "0.13.2", default-features = false }
 object_store_opendal = "0.58.0"
 once_cell = "1.21.4"
-onpair = "0.2.0"
+onpair = "0.2.1"
 opendal = { version = "0.58.1", default-features = false }
 opentelemetry = "0.32.0"
 opentelemetry-otlp = "0.32.0"
diff --git a/encodings/onpair/src/compress.rs b/encodings/onpair/src/compress.rs
index e2cf69353ec..1b0f4c68d18 100644
--- a/encodings/onpair/src/compress.rs
+++ b/encodings/onpair/src/compress.rs
@@ -4,6 +4,7 @@
 //! Train + compress entry points for the OnPair encoding.
 
 use onpair::Config;
+use onpair::Rows;
 use vortex_array::ArrayRef;
 use vortex_array::ExecutionCtx;
 use vortex_array::IntoArray;
@@ -19,7 +20,6 @@ use vortex_buffer::ByteBuffer;
 use vortex_buffer::ByteBufferMut;
 use vortex_error::VortexExpect;
 use vortex_error::VortexResult;
-use vortex_error::vortex_err;
 use vortex_mask::AllOr;
 
 use crate::OnPair;
@@ -43,14 +43,8 @@ pub fn onpair_compress(
     }
 
     let views = array.views();
-    let flat_bytes: usize = views.iter().map(|v| v.len() as usize).sum();
-
-    // TODO(francesco): we flatten because onpair training needs a contiguous `(bytes, offsets)`
-    // pair. Allowing onpair to train on a slice-of-slices would let us skip this copy.
-    let mut flat: Vec<u8> = Vec::with_capacity(flat_bytes);
-    let mut offsets: Vec<u64> = Vec::with_capacity(len + 1);
-    let mut uncompressed_lengths: BufferMut<i32> = BufferMut::with_capacity(len);
-    offsets.push(0);
+    let mut uncompressed_lengths: BufferMut<u32> = BufferMut::zeroed(len);
+    let mut total_bytes = 0usize;
     let buffers = array
         .data_buffers()
         .as_ref()
@@ -60,33 +54,33 @@ pub fn onpair_compress(
 
     match mask.bit_buffer() {
         AllOr::All => {
-            for view in views {
-                let bytes = view_bytes(view, &buffers);
-                flat.extend_from_slice(bytes);
-                offsets.push(u64::try_from(flat.len()).vortex_expect("offset must fit in u64"));
-                uncompressed_lengths
-                    .push(i32::try_from(view.len()).vortex_expect("must fit in i32"));
+            for (view, length) in views.iter().zip(uncompressed_lengths.iter_mut()) {
+                *length = view.len();
+                total_bytes += *length as usize;
             }
         }
         AllOr::None => unreachable!("all-null input handled above"),
         AllOr::Some(validity) => {
-            for (view, valid) in views.iter().zip(validity.iter()) {
+            for ((view, length), valid) in views
+                .iter()
+                .zip(uncompressed_lengths.iter_mut())
+                .zip(validity.iter())
+            {
                 if valid {
-                    let bytes = view_bytes(view, &buffers);
-                    flat.extend_from_slice(bytes);
-                    offsets.push(u64::try_from(flat.len()).vortex_expect("offset must fit in u64"));
-                    uncompressed_lengths
-                        .push(i32::try_from(view.len()).vortex_expect("must fit in i32"));
-                } else {
-                    offsets.push(u64::try_from(flat.len()).vortex_expect("offset must fit in u64"));
-                    uncompressed_lengths.push(0);
+                    *length = view.len();
+                    total_bytes += *length as usize;
                 }
             }
         }
     }
 
-    let column = onpair::compress(&flat, &offsets, config)
-        .map_err(|e| vortex_err!("OnPair compress failed: {e}"))?;
+    let rows = ViewRows {
+        views,
+        buffers: &buffers,
+        lengths: uncompressed_lengths.as_slice(),
+        total_bytes,
+    };
+    let column = onpair::compress_rows::<_, u64>(&rows, config);
     let (dict, codes, row_offsets) = column.into_raw();
     let (dict_bytes, dict_offsets) = dict.into_raw();
     let codes_offsets = codes_offsets_array(&row_offsets);
@@ -113,12 +107,34 @@ pub fn onpair_compress(
     Ok(encoded.into_array())
 }
 
-fn view_bytes<'a>(view: &'a BinaryView, buffers: &'a [&ByteBuffer]) -> &'a [u8] {
-    if view.is_inlined() {
-        view.as_inlined().value()
-    } else {
-        let view_ref = view.as_view();
-        &buffers[view_ref.buffer_index as usize][view_ref.as_range()]
+/// Reads inline and external values in place, treating null rows as empty.
+struct ViewRows<'a> {
+    views: &'a [BinaryView],
+    buffers: &'a [&'a ByteBuffer],
+    lengths: &'a [u32],
+    total_bytes: usize,
+}
+
+impl Rows for ViewRows<'_> {
+    fn num_rows(&self) -> usize {
+        self.views.len()
+    }
+
+    fn total_bytes(&self) -> usize {
+        self.total_bytes
+    }
+
+    #[inline]
+    fn row(&self, i: usize) -> &[u8] {
+        let view = &self.views[i];
+        if self.lengths[i] == 0 {
+            &[]
+        } else if view.is_inlined() {
+            view.as_inlined().value()
+        } else {
+            let view_ref = view.as_view();
+            &self.buffers[view_ref.buffer_index as usize][view_ref.as_range()]
+        }
     }
 }