Of the 13 most widely spoken languages of India, OS X comes with built-in support for 6 of them via its Devanagari, Gujarati, Gurmukhi, and Tamil fonts and keyboards. You can find info on support for another three (Telugu, Kannada, and Malayalam) in my earlier note here. Below is info on using the remaining 4 -- Bengali, Urdu, Oriya, and Assamese.
For Bangla/Bengali, you can download OS X fonts and keyboard layout from this site. The Bengali script is also used for Assamese.
For Urdu, OS X comes with the required Arabic script font, but you need to install a keyboard layout. One can be obtained here or from my iDisk.
For Oriya, you will need to download a font like Utkal listed at this site. Correct display is only possible in OpenOffice/X11. A (very) expermental keyboard layout is on my iDisk.