I dont know anything about objectStore but I have built transaction
managers.

class ConnectionManager {
  public static ConnectionManager getSingleton();
  public Connection getNamedConnection(String name);
  public void beginTransaction();
  public void endTransaction(boolean commitp);
}
ConnectionManager.getSingleton().beginTransaction();
boolean commitp = false;
try {
  Connection conn =
    ConnectionManager.getSingleton().getNamedConnection("odi:pool");
  /* do stuff */
  commitp = true;
}
finally {
  // commitp is false on exception, so votes to rollback.
  ConnectionManager.getSingleton().endTransaction(commitp);
}

Store the transaction object in a thread local.  beginTransaction
increments a count so the begin/ends can nest.  endTransaction
decrements the counter.  When it goes to zero, if any endTransaction
or any Connection voted against commit, then rollback.

getNamedConnection ensures that a connection from the named pool
lives in the transaction.  All connections in the transaction
are committed together or rolled back together in a two-phase
commit.  Theres some risk that the commits fail, but its about
as good as you can do without a real XA transaction.