Update. I'm not 100% certain, but I don't think the Mets can offer Cespedes a QO if he opts out after 2016. Why? Because the CBA provides that process (QO, draft pick compensation) for players who become free agents because they have six years' major-league service time. Cespedes will not have six years service time as of the end of 2016 - he's at 4.000 years now, and will presumably be at 5.000 years come October.
I obviously don't know the specific language of Cespedes' contract, but under the general terms of the CBA, if he opts out of the remainder of his contract this fall, the Mets would nonetheless still have rights to him, and he would be eligible for salary arbitration (for a one-year contract with the Mets) - unless the Mets non-tender him. The terms of the "opt out" clause in his contract may be that the Mets have agreed to non-tender Cespedes if he takes the "opt out" - in which case he would become an unrestricted free agent, but with no draft pick compensation to the Mets.