Would need more information - also, the flight must be for at least 28 days in advance, so if you are travelling on Monday 1 June, this offer wont apply.
Unless rules have changed, you will have awkward questions to answer (if not need a VISA) to enter the USA on a one way ticket.
Where are you flying from, when are you travelling (out and back), any flexibility on dates/times of flight?
A good travel agent should be able to find you the best deal.