diff --git a/README.md b/README.md
index dc5e17b..734f7c7 100644
--- a/README.md
+++ b/README.md
@@ -29,5 +29,5 @@ Source: https://www.programmfabrik.de/en/assignment-frontend-backend-developer-j
 ## Time spent
 
 * ~0.5 hours to set up the project;
-* ~4.5 hours to implement game board serialization, tic-tac-toe rules and game solver (with tests);
+* ~5 hours to implement game board serialization, tic-tac-toe rules and game solver (with tests);
 * ...
diff --git a/src/lib/board.ts b/src/lib/board.ts
index eff0082..6a0d4a0 100644
--- a/src/lib/board.ts
+++ b/src/lib/board.ts
@@ -1,7 +1,7 @@
-import { SquareState } from "./datatypes.ts";
+import { BoardType, SquareState } from "./datatypes.ts";
 import { unreachable } from "./utils.ts";
 
-export class Board {
+export class Board implements BoardType {
   // State should be immutable
   constructor(private readonly state: SquareState[][]) {}
 
diff --git a/src/lib/datatypes.ts b/src/lib/datatypes.ts
index 04c64d2..d86dab2 100644
--- a/src/lib/datatypes.ts
+++ b/src/lib/datatypes.ts
@@ -39,6 +39,10 @@ export type GameRules = {
   getBoardOutcome(board: BoardType): CurrentOutcome;
 };
 
+export type Opponent = {
+  getNextMove(board: BoardType, currentPlayer: Player): BoardType;
+};
+
 export const getExpectedOutcomeByCurrentOutcome = (
   currentOutcome: Exclude<CurrentOutcome, CurrentOutcome.Undecided>,
 ): ExpectedOutcome => {
diff --git a/src/lib/opponent.spec.ts b/src/lib/opponent.spec.ts
index 4fd5ee2..14d3aae 100644
--- a/src/lib/opponent.spec.ts
+++ b/src/lib/opponent.spec.ts
@@ -1,7 +1,7 @@
 import t from "tap";
+import { Board } from "./board.ts";
 import { ExpectedOutcome, FinalOutcome, Player } from "./datatypes.ts";
 import { createOpponent } from "./opponent.ts";
-import { Board } from "./board.ts";
 
 void t.test("createOpponent", async (t) => {
   const outcomesByBoard: Record<string, ExpectedOutcome> = {
diff --git a/src/lib/opponent.ts b/src/lib/opponent.ts
index e278636..949a21f 100644
--- a/src/lib/opponent.ts
+++ b/src/lib/opponent.ts
@@ -1,7 +1,7 @@
-import { BoardType, ExpectedOutcome, Player, SquareState, getOccupiedStateByPlayer } from "./datatypes.ts";
+import { ExpectedOutcome, Opponent, SquareState, getOccupiedStateByPlayer } from "./datatypes.ts";
 
-export const createOpponent = (outcomesByBoard: Map<string, ExpectedOutcome>) => {
-  const getNextMove = (board: BoardType, currentPlayer: Player) => {
+export const createOpponent = (outcomesByBoard: Map<string, ExpectedOutcome>): Opponent => {
+  const getNextMove: Opponent["getNextMove"] = (board, currentPlayer) => {
     const currentExpectedOutcome = outcomesByBoard.get(board.serialize());
     if (!currentExpectedOutcome) {
       throw new Error(`Board is not solved: ${board.serialize()}`);
diff --git a/src/lib/tictactoe.test.ts b/src/lib/tictactoe.test.ts
index 6866347..7892896 100644
--- a/src/lib/tictactoe.test.ts
+++ b/src/lib/tictactoe.test.ts
@@ -1,6 +1,8 @@
-import t from "tap";
+import t, { Test } from "tap";
 
-import { ExpectedOutcome, FinalOutcome } from "./datatypes.ts";
+import { Board } from "./board.ts";
+import { ExpectedOutcome, FinalOutcome, Opponent, Player } from "./datatypes.ts";
+import { createOpponent } from "./opponent.ts";
 import { computeAllSolutions } from "./solver.ts";
 import { rules } from "./tictactoe-rules.ts";
 
@@ -142,3 +144,70 @@ void t.test("computeAllSolutions", async (t) => {
     "OO_|__X|_XX": { finalOutcome: FinalOutcome.WinO, movesLeft: 1 },
   });
 });
+
+void t.test("createOpponent", async (t) => {
+  const checkNextMove = (
+    t: Test,
+    opponent: Opponent,
+    currentBoardSerialized: string,
+    currentPlayer: Player,
+    expectedNextBoardSerialized: string,
+  ) => {
+    t.equal(
+      opponent.getNextMove(Board.fromSerialized(currentBoardSerialized), currentPlayer).serialize(),
+      expectedNextBoardSerialized,
+    );
+  };
+
+  void t.test("1x5 board", async (t) => {
+    const outcomes = computeAllSolutions(1, 5, rules);
+    const opponent = createOpponent(outcomes);
+
+    checkNextMove(t, opponent, "_____", Player.X, "X____");
+    checkNextMove(t, opponent, "X____", Player.O, "XO___");
+    checkNextMove(t, opponent, "XO___", Player.X, "XOX__");
+    checkNextMove(t, opponent, "XOX__", Player.O, "XOXO_");
+    checkNextMove(t, opponent, "XOXO_", Player.X, "XOXOX");
+
+    checkNextMove(t, opponent, "__X__", Player.O, "_OX__");
+    checkNextMove(t, opponent, "_OX__", Player.X, "XOX__");
+    checkNextMove(t, opponent, "XOX__", Player.O, "XOXO_");
+    checkNextMove(t, opponent, "XOXO_", Player.X, "XOXOX");
+
+    checkNextMove(t, opponent, "O_X__", Player.X, "O_XX_");
+    checkNextMove(t, opponent, "O_XX_", Player.O, "OOXX_");
+    checkNextMove(t, opponent, "OOXX_", Player.X, "OOXXX");
+  });
+
+  void t.test("3x3 board", async (t) => {
+    const outcomes = computeAllSolutions(3, 3, rules);
+    const opponent = createOpponent(outcomes);
+
+    checkNextMove(t, opponent, "___|___|___", Player.X, "X__|___|___");
+    checkNextMove(t, opponent, "X__|___|___", Player.O, "X__|_O_|___");
+    checkNextMove(t, opponent, "X__|_O_|___", Player.X, "XX_|_O_|___");
+    checkNextMove(t, opponent, "XX_|_O_|___", Player.O, "XXO|_O_|___");
+    checkNextMove(t, opponent, "XXO|_O_|___", Player.X, "XXO|_O_|X__");
+    checkNextMove(t, opponent, "XXO|_O_|X__", Player.O, "XXO|OO_|X__");
+    checkNextMove(t, opponent, "XXO|OO_|X__", Player.X, "XXO|OOX|X__");
+    checkNextMove(t, opponent, "XXO|OOX|X__", Player.O, "XXO|OOX|XO_");
+    checkNextMove(t, opponent, "XXO|OOX|XO_", Player.X, "XXO|OOX|XOX");
+
+    checkNextMove(t, opponent, "___|___|__X", Player.O, "___|_O_|__X");
+    checkNextMove(t, opponent, "___|_O_|__X", Player.X, "X__|_O_|__X");
+    checkNextMove(t, opponent, "X__|_O_|__X", Player.O, "XO_|_O_|__X");
+    checkNextMove(t, opponent, "XO_|_O_|__X", Player.X, "XO_|_O_|_XX");
+    checkNextMove(t, opponent, "XO_|_O_|_XX", Player.O, "XO_|_O_|OXX");
+    checkNextMove(t, opponent, "XO_|_O_|OXX", Player.X, "XOX|_O_|OXX");
+    checkNextMove(t, opponent, "XOX|_O_|OXX", Player.O, "XOX|_OO|OXX");
+    checkNextMove(t, opponent, "XOX|_OO|OXX", Player.X, "XOX|XOO|OXX");
+
+    checkNextMove(t, opponent, "XO_|___|___", Player.X, "XO_|X__|___");
+    checkNextMove(t, opponent, "XO_|X__|___", Player.O, "XO_|X__|O__");
+    checkNextMove(t, opponent, "XO_|X__|O__", Player.X, "XO_|XX_|O__");
+    checkNextMove(t, opponent, "XO_|XX_|O__", Player.O, "XOO|XX_|O__");
+    checkNextMove(t, opponent, "XOO|XX_|O__", Player.X, "XOO|XXX|O__");
+
+    checkNextMove(t, opponent, "XO_|XXO|O__", Player.X, "XO_|XXO|O_X");
+  });
+});