diff --git a/src/auth/routes.py b/src/auth/routes.py
index 1de1ce3..6c23744 100644
--- a/src/auth/routes.py
+++ b/src/auth/routes.py
@@ -16,6 +16,25 @@ MIN_PASSWORD_LENGTH = 8
 _DUMMY_HASH = generate_password_hash("dummy-password-for-timing")
 
 
+def resolve_oauth_user(
+    provider: str, subject: str, email: str, email_verified: bool
+) -> User:
+    identity = db.get_oauth_identity(provider, subject)
+    if identity is not None:
+        return User(db.get_user_by_id(identity["user_id"]))
+
+    row = db.get_user_by_email(email)
+    if row is not None:
+        db.link_oauth_identity(provider, subject, row["id"])
+        if email_verified and not row["email_verified"]:
+            db.set_email_verified(row["id"])
+        return User(db.get_user_by_id(row["id"]))
+
+    user_id = db.create_oauth_user(email)
+    db.link_oauth_identity(provider, subject, user_id)
+    return User(db.get_user_by_id(user_id))
+
+
 def _redirect_with_error(path: str, error: str, email: str | None = None):
     url = f"{path}?error={error}"
     if email:
diff --git a/tests/auth/test_oauth_resolve.py b/tests/auth/test_oauth_resolve.py
new file mode 100644
index 0000000..90ddfdd
--- /dev/null
+++ b/tests/auth/test_oauth_resolve.py
@@ -0,0 +1,53 @@
+from werkzeug.security import generate_password_hash
+
+from src.auth import db
+from src.auth.models import User
+from src.auth.routes import resolve_oauth_user
+
+
+def test_creates_new_user_when_unknown(users_db_path):
+    db.init_schema()
+    user = resolve_oauth_user("linkedin", "sub-1", "new@example.com", True)
+    assert isinstance(user, User)
+    row = db.get_user_by_id(int(user.get_id()))
+    assert row["email"] == "new@example.com"
+    assert row["password_hash"] is None
+    assert row["email_verified"] == 1
+    assert db.get_oauth_identity("linkedin", "sub-1")["user_id"] == row["id"]
+
+
+def test_links_to_existing_email_account(users_db_path):
+    db.init_schema()
+    uid = db.create_user("alice@example.com", generate_password_hash("password12"))
+    db.set_email_verified(uid)
+
+    user = resolve_oauth_user("linkedin", "sub-2", "alice@example.com", True)
+    assert int(user.get_id()) == uid
+    assert db.get_oauth_identity("linkedin", "sub-2")["user_id"] == uid
+    # Le compte garde son mot de passe.
+    assert db.get_user_by_id(uid)["password_hash"] is not None
+
+
+def test_links_and_verifies_unverified_existing_account(users_db_path):
+    db.init_schema()
+    uid = db.create_user("bob@example.com", generate_password_hash("password12"))
+    assert db.get_user_by_id(uid)["email_verified"] == 0
+
+    resolve_oauth_user("linkedin", "sub-3", "bob@example.com", True)
+    assert db.get_user_by_id(uid)["email_verified"] == 1
+
+
+def test_returns_same_user_for_known_identity(users_db_path):
+    db.init_schema()
+    first = resolve_oauth_user("linkedin", "sub-4", "carol@example.com", True)
+    second = resolve_oauth_user("linkedin", "sub-4", "carol@example.com", True)
+    assert first.get_id() == second.get_id()
+    # Pas de doublon d'identité.
+    count = (
+        db.get_conn()
+        .execute(
+            "SELECT COUNT(*) FROM oauth_identities WHERE provider='linkedin' AND subject='sub-4'"
+        )
+        .fetchone()[0]
+    )
+    assert count == 1