Light:  
  • Oncidiums
    prefer bright light. An eastern, western, or southern exposure is
    ideal. If the leaves are dark green, the plants is not getting enough
    light. On the other hand, if the leaves have a reddish color, they are
    getting too much light.    
  • Oncidiums do best in an east, west, or south facing window.
  • If
    the foliage on the plant is dark green and limp, it is an indication
    the plant needs more light. Reddish leaves indicate the plant is getting
    to much light. Ideally, the foliage should be a bright olive green.
Temperature:
  • Ideally,o ncidiums prefer night time temperatures between 55-65°F and 75-85°F
    during the day. However, oncidiums are fairly tough in the sense that
    they can take temperatures up to 95°F temperature. If the temperatures
    do get little high, one should increase the humidity, water and air
    circulation around the plant to compensate.  
  • Do not expose oncidiums to cold drafts. 
Water:  
  • Oncidiums do not need as much water as phalaenopsis. They have water storage bulbs and need to dry out between waterings.
  • The
    best way to water oncidiums is to remove them from their decorative
    pots and flush them with water for 1 to 2 minutes under a faucet. If
    that is not possible, you can also place 5 or so ice cubes on the bark
    once a week. 
Humidity:  
  • The
    ideal humidity conditions for oncidiums is between 55% and 75%
    humidity. In warmer and drier conditions, oncidiums can be placed on
    gravel that has water in the bottom to elevate the local humidity.
    However, you should never let the pots sit in water as this will cause
    root rot. 
  • On days of high humidity make sure there is good air circulation around the plants. 
Feeding:  
  • In general, oncidiums do best and are at their healthiest when fertilized on a regular basis. Once a month is ideal. 
  • The
    growers recommend that you use a well balanced formulation such as
    30-10-10 or a ratio that is similar. However, it is important that the
    plants are slightly moist before fertilizing to avoid burning.