Soak the root ball for 15 minutes. Then, using a steak knife, cut about a one to two inch deep slice across the bottom of the roots. This will of course damage some roots, but with that huge root ball your orchid will be fine. Make the cut as small as possible. If your orchid is in bloom - do not do this as this disturbance will make the blooms/buds fall off.

Now, after making the cut, grab each side and gently pull apart until you are able to remove the grower's plug (the sphag ball in the center.) This is the part that will rot your orchid's roots. Once that sphag plug is removed, repot in a bark mix made for oncidiums. A little sphag on the outsides of the root ball will not hurt much, just get the bulk of it out. Happy repotting. (And if you want to know if you have a Sharry Baby or a Colmanara Wild Cat, post a photo of the blooms.

Cheers,
BD