The beta and production OS requirements have remained the same for the managed provider. Windows 7 was the earliest client OS Oracle supported for managed ODP.NET.
At a theoretical level, managed ODP.NET can run on any OS that .NET 4 can run on. At a practical level, platform combinations need to be tested and certified to ensure they are in proper working order. As testing resources are finite, a choice needs to be made to test on all currently supported OSes or focus on the most popular ones. In the former case, you spread your testing out more thinly. In the latter, you can focus your energies more on the popular OSes to ensure higher quality running on those OSes. Since most OS industry analysts expect Windows XP and Vista to quickly recede in market share compared to Windows 7 and 8.x, the best choice appeared to focus on Windows 7 and higher.
XP will reach Microsoft's end of life middle next year. Very few business users will adopt it for the next iteration of ODP.NET apps.
Vista has low popularity, already below 5% in the overall Windows OS share and quickly declining. In the business community, most jumped from XP to 7, skipping Vista. The Vista market share among business users is much likely even lower as the 5% number also includes consumers.