diff --git a/packages/messaging/lib/messages/pre-167/OracleAttestation.ts b/packages/messaging/lib/messages/pre-167/OracleAttestation.ts
index 73056d1b..45ad9cd4 100644
--- a/packages/messaging/lib/messages/pre-167/OracleAttestation.ts
+++ b/packages/messaging/lib/messages/pre-167/OracleAttestation.ts
@@ -3,13 +3,14 @@ import assert from 'assert';
 import { math, verify } from 'bip-schnorr';
 
 import { MessageType } from '../../MessageType';
+import { IDlcMessage } from '../DlcMessage';
 import {
   deserializeTlv,
   ITlv,
   serializeTlv,
 } from '../../serialize/deserializeTlv';
 import { getTlv } from '../../serialize/getTlv';
-import { IDlcMessage } from '../DlcMessage';
+import { getBigSizeBytesLength } from '../../utils/bigSize';
 
 /**
  * In order to make it possible to hold oracles accountable in cases where
@@ -58,13 +59,19 @@ export class OracleAttestationV0Pre167 implements IDlcMessage {
     }
 
     let deserializedBytes =
-      2 + Number(eventIdLength) + 32 + 2 + numSignatures * 64; // Count 2 bytes for eventIdLength + eventIdLength
-    // + 32 bytes for oraclePubkey + 2 bytes for numSignatures + 64 bytes per signature
+      getBigSizeBytesLength(eventIdLength) + // bytes length of eventIdLength
+      Number(eventIdLength) + // bytes length of eventId
+      32 + // oraclePubKey is 32 bytes
+      2 + // numSignatures is a uint16 (2 bytes)
+      numSignatures * 64; // each signature is 64 bytes
+
     while (deserializedBytes < instance.length) {
       const outcomeLen = reader.readBigSize();
       const outcomeBuf = reader.readBytes(Number(outcomeLen));
       instance.outcomes.push(outcomeBuf.toString());
-      deserializedBytes += 2 + Number(outcomeLen); // Count 2 bytes for outcomeLen + outcomeLen
+      deserializedBytes +=
+        getBigSizeBytesLength(outcomeLen) + // bytes length of outcomeLen
+        Number(outcomeLen); // bytes length of outcomeBuf
     }
 
     while (!reader.eof) {
diff --git a/packages/messaging/lib/utils/bigSize.ts b/packages/messaging/lib/utils/bigSize.ts
new file mode 100644
index 00000000..87eda040
--- /dev/null
+++ b/packages/messaging/lib/utils/bigSize.ts
@@ -0,0 +1,15 @@
+// BigSize integers are deserialized using @node-lightning/bufio BufferReader.readBigSize()
+// Refer to https://github.com/node-lightning/node-lightning/blob/e9b349b200997b8c5e46751ae6eb13a283323f5b/packages/bufio/lib/BufferReader.ts#L194
+
+/**
+ * Extract the size of a deserialized bigSize integer
+ * @param val
+ * @returns The size in bytes of the value parameter
+ */
+
+export const getBigSizeBytesLength = (val: bigint): number => {
+  if (val < BigInt('0xfd')) return 1;
+  if (val < BigInt('0x10000')) return 3;
+  if (val < BigInt('0x100000000')) return 5;
+  return 9;
+};