As a general rule, Oncidiums like to be pot bound in order to bloom well. They will need to be repotted when they are literally climbing out of the pot that they’re in. For those who have not seen this, the plant will grow over the side of the pot and start sending roots into the air. Most sympodial orchids will do this once they have outgrown the pot that they’re in.
Oncidiums should be repotted in the spring when the previous year’s growth is starting to mature. Most plants should be repotted in medium grade media rather than a traditional orchid mix since these plants have fine roots rather than the more common large roots that most orchids have. It is a good idea to add a bit of charcoal to the mix in order to keep the mix from deteriorating too quickly.
Remove the plant from its container and wash off all the roots being sure to eliminate any media attached to them. Hold the plant and roots up to the new container and determine what length will be about three-quarters into the new pot and then clip all the roots that will be going in the pot to encourage new growth. As with all sympodials, put the oldest growth against the side of the pot with the new growth positioned toward the other side of the pot so that there is lots of room for growth.
Any medium you use for repotting your orchids should be soaked for approximately 24 hours prior to use. It needs to have enough time to absorb water so that the newly planted roots are not desiccated. Prepare the pot with drainage material at the bottom of the pot, and place a portion of medium over it at the bottom. Place your plant into the new pot and then carefully pour the medium around the roots. Tamp down carefully with your fingers, or a bamboo stake. I like to put bamboo stakes or shish kabob skewers on two or three sides of a plant to hold it in place until the plant has taken hold and becomes established in the pot.
Put the plant in a warmer and lower light position than it had been growing in before to encourage it to grow new roots. If possible provide higher than normal humidity and do not water until new root growth is evident. Then move to its normal position in about a month.