The ICO format is the native icon format under OS/2 and Windows, Note that the Windows icon format is significantly different from OS/2's icon format. Thus it is very important that you choose the right format when you save icons.
The default extension is ICO.
Supporting Platforms and Applications
Intel machines running OS/2
PMView is able to read all types of OS/2 icons; The old "Icon" and the new "Color Icon" format can be read.