diff --git a/README.md b/README.md
index 95d657f..660705a 100644
--- a/README.md
+++ b/README.md
@@ -50,6 +50,44 @@ Installing this plugin adds support for the Optimism ecosystem:
 ape console --network optimism:sepolia
 ```
 
+### OP Stack
+
+Use the `optimism` base-class in any custom networks using the OP stack.
+For example, to configure a custom network for Fraxtal network, add the following to your `pyproject.toml`
+
+```toml
+[[tool.ape.networks.custom]]
+name = "mainnet"
+ecosystem = "fraxtal"
+# Tell Ape to use optimism as the base plugin, instead of ethereum.
+base_ecosystem_plugin = "optimism"
+chain_id = 252
+
+# (optional): Configure an RPC. Else, Ape will select a random one automatically.
+[tool.ape.node.fraxtal.mainnet]
+uri = "https://rpc.frax.com"
+```
+
+Or equivalent `ape-config.yaml`:
+
+```yaml
+networks:
+  custom:
+    - name: mainnet
+      ecosystem: fraxtal
+      base_ecosystem_plugin: optimism
+
+node:
+ fraxtal:
+    mainnet:
+      uri: https://rpc.frax.com
+```
+
+There are two main benefits of using Optimism as the base-class instead of Ethereum for networks using the OP stack:
+
+1. **Closer defaults**: The block time default is `2` for Optimism networks, which may be a better default value than Ethereum's higher block time parameters.
+2. **Existence of System Transactions**: The Optimism base-class is aware of system transactions, which are transactions invoked by the sequencer.
+
 ## Development
 
 Comments, questions, criticisms and pull requests are welcomed.
diff --git a/ape_optimism/ecosystem.py b/ape_optimism/ecosystem.py
index 5639dfc..6b1a825 100644
--- a/ape_optimism/ecosystem.py
+++ b/ape_optimism/ecosystem.py
@@ -1,4 +1,4 @@
-from typing import TYPE_CHECKING, cast
+from typing import cast
 
 from ape.api import TransactionAPI
 from ape.exceptions import ApeException, APINotImplementedError
@@ -8,9 +8,7 @@
     NetworkConfig,
     create_network_config,
 )
-
-if TYPE_CHECKING:
-    from eth_pydantic_types import HexBytes
+from eth_pydantic_types import HexBytes
 
 NETWORKS = {
     # chain_id, network_id
@@ -34,16 +32,30 @@ class OptimismConfig(BaseEthereumConfig):
 
 class SystemTransaction(TransactionAPI):
     type: int = SYSTEM_TRANSACTION
+    _hash: HexBytes
+
+    def __init__(self, *args, **kwargs):
+        hash = kwargs.pop("hash", None)
+        super().__init__(*args, **kwargs)
+        self._hash = hash
 
     @property
     def txn_hash(self) -> "HexBytes":
-        raise APINotImplementedError("Unable to calculate the hash of system transactions.")
+        return self._hash
 
     def serialize_transaction(self) -> bytes:
         raise APINotImplementedError("Unable to serialize system transactions.")
 
 
 class Optimism(Ethereum):
+    """
+    A base class for Optimism or OP-stack networks.
+    The main differences from Ethereum are:
+
+    1. Different network defaults (block time is only '2', by default).
+    2. Recognition of "System transactions" (type 126).
+    """
+
     @property
     def config(self) -> OptimismConfig:  # type: ignore
         return cast(OptimismConfig, self.config_manager.get_config("optimism"))
diff --git a/tests/test_ecosystem.py b/tests/test_ecosystem.py
index 1d47b6c..0b77236 100644
--- a/tests/test_ecosystem.py
+++ b/tests/test_ecosystem.py
@@ -1,6 +1,7 @@
 import pytest
 from ape_ethereum.transactions import TransactionType
 from ethpm_types.abi import MethodABI
+from hexbytes import HexBytes
 
 from ape_optimism.ecosystem import SYSTEM_TRANSACTION, SystemTransaction
 
@@ -45,6 +46,7 @@ def test_create_transaction_type_2(optimism, tx_kwargs):
 
 
 def test_create_transaction_type_126(optimism):
+    txn_hash = "0x417b3339801514042a4b3fa24658931438389996b29d0749cc7555a13a9ad89a"
     data = {
         "chainId": 0,
         "to": "0x4200000000000000000000000000000000000015",
@@ -55,10 +57,12 @@ def test_create_transaction_type_126(optimism):
         "data": "0x",
         "type": 126,
         "accessList": [],
+        "hash": HexBytes(txn_hash),
     }
     actual = optimism.create_transaction(**data)
     assert isinstance(actual, SystemTransaction)
     assert actual.type == SYSTEM_TRANSACTION
+    assert actual.txn_hash == HexBytes(txn_hash)
 
 
 @pytest.mark.parametrize(