It is recommended to support one complete Windows 8-bit codepage in each font. Since Indic scripts do not have their own 8-bit codepage in Windows, but instead use only Unicode, the normal practice is to include support for the Latin-1 codepage 1252.
A font with appropriate Devanagari layout features built in VOLT will work fine in Windows 7. Note that you can use only the more recent <dev2> script tag and shaping model if you are targeting only Windows Vista and later; for backwards compatibility with older versions of Windows, you will need to also (or instead) implement the deprecated <deva> tag and shaping model. [http://www.microsoft.com/typography/SpecificationsOverview.mspx]See the Microsoft Devanagari spec[/http://www.microsoft.com/typography/SpecificationsOverview.mspx].
I am not sure about the support for Indic OT fonts in Mac OSX. If supported at all, it would only be in the most recent versions.