One of the most prominent threats in Dreamworld OU, Keldeo definitely is one to prepare a team for. Although Keldeo has a limited movepool, they are enough to get the job done. With access to Hydro Pump Keldeo can muscle through the bulkiest of special walls. This insane power is further boosted via Calm Mind, Choice Specs and the abundant rain provided by Politoed. Keldeo also has access to a great move in Secret Sword, able to hit Pokemon like Blissey in their weaker defensive stats. Rounding out Keldeo's coverage is the choice between Hidden Power Ghost or Hidden Power Ice, covering either Jellicent, Latios and Latias or Dragonite, Salamence and the aforementioned Latis. Pending on the Hidden Power the opposite mons can easily wall Keldeo, and either toxic it or beat it down offensively. Keldeo is also stopped by strong attacks from faster pokemon like Thundurus and Tornadus who can abuse the rain to fire off their STABs, both of which are super-effective against Keldeo.
I am on my phone so please forgive mistakes I've made (and the missing sprite). I am also gonna snag Blaziken Deoxys-S, Thundurus, Manaphy and Excadrill for when I get home or at a comp in school.
EDIT: thanks harsha!