diff --git a/packages/ocproto/lib/ocapn.capnp b/packages/ocproto/lib/ocapn.capnp
new file mode 100644
index 0000000..e28829d
--- /dev/null
+++ b/packages/ocproto/lib/ocapn.capnp
@@ -0,0 +1,53 @@
+# ocapn.capnp
+@0xcd301da1d95b8242;
+
+struct Value {
+  union {
+    ## Atoms ##
+
+    undefined @0 :Void;
+    null @1 :Void;
+    bool @2 :Bool;
+    float64 @3 :Float64;
+
+    unsignedInt @4 :Data;
+    # Non-negative integer, in big-endian format
+    negativeInt @5 :Data;
+    # Negative integer. value is `-1 - n`, where `n`
+    # is the data interpreted as an unsigned big-endian integer.
+
+    string @6 :Text;
+    byteString @7 :Data;
+    symbol @8 :Text; # Might be removed, pending #46.
+
+    ## Containers ##
+
+    list @9 :List(Value);
+
+    struct @10 :List(StructField);
+    # Duplicate keys are not allowed; will need to enforce this at a higher level
+    # of abstraction.
+
+    tagged :group {
+      label @11 :Text;
+      value @12 :Value;
+    }
+
+    capability @13 :Cap;
+
+    error @14 :Error;
+  }
+}
+
+struct StructField {
+  key @0 :Text;
+  value @1 :Value;
+}
+
+interface Cap {
+  # TODO
+}
+
+struct Error {
+  # TODO, pending #10.
+}
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