Might need more light also - how far from the window is it? All of my indoors orchids are on the window sills. Oncidium types should do well with a bit of morning or afternoon sun
Also I water mine here in the bay area more than once a week in summer, so in your very dry conditions it may need more water.
I have bamboo skewers in all my pots to help determine when to water
good luck and let us know how it goes