I would also go with WebGL, because it's the standard from the Khronos group since February. Firefox got it for almost one year (but still in version 4 beta / rc). Google Chrome got it in the current version. Opera just implemented it (I think not quite yet) http://my.opera.com/core/blog/2011/02/28/webgl-and-hardware-acceleration-2
As yojimbo87 will tell you that Microsoft is not a member of the Khronos group because it supports Microsoft DirectX, which is in concurrency with OpenGL.
Regarding O3D, Google used it as an API plugin, but on the main web page you can see that it is now deprecated. http://code.google.com/intl/fr/apis/o3d/
For another way to do 3D on the Internet, you can use something like VRML / X3D, flash got something serious, but here is a good thing that happens with WebGL: You have WebGL implemented in firefox for a mobile phone (which will be available soon) ) I think it wonβt take long until Chrome and Safari do the same.
If you are not convinced yet, WebGL is hardware accelerated, you can use the shader inside. Many APIs / tools appear. A great website to find out and get some news about WebGL: http://learningwebgl.com/blog/
Good luck with it.
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