You are right that orfeo toolbox is not yet available in the 64 bit Version of OSGeo4w installer. You can compare the package listings here:

http://download.osgeo.org/osgeo4w/x86/versions.html

http://download.osgeo.org/osgeo4w/x86_64/versions.html

The 64-bit version is still experimental, with packages missing or not working as expected.

However, it is possible to use the 32-bit installer to install orfeo toolbox in C:\OSGeo4W (while the 64bit QGIS is in C:\OSGeo4w64), and nethertheless enter the paths to otb in 64-bit QGIS 2:

C:\OSGeo4W\apps\orfeotoolbox\applications
C:\OSGeo4W\bin

This even works with the standalone installers.

You are right that orfeo toolbox is not yet available in the 64 bit Version of OSGeo4w installer. You can compare the package listings here:

http://download.osgeo.org/osgeo4w/x86/versions.html

http://download.osgeo.org/osgeo4w/x86_64/versions.html

The 64-bit version is still experimental, with packages missing or not working as expected.

However, it is possible to use the 32-bit installer to install orfeo toolbox in C:\OSGeo4W (while the 64bit QGIS is in C:\OSGeo4w64), and nethertheless enter the paths to otb in 64-bit QGIS 2:

C:\OSGeo4W\apps\orfeotoolbox\applications
C:\OSGeo4W\bin

This even works with the standalone installers.

UPDATE

According to http://download.osgeo.org/osgeo4w/versions_diff-x86-x86_64.html both 32bit and 64 bit version of OSGEO4W now include the Orfeo Toolbox

The paths have to be entered in Processing -> Options -> Providers -> Orfeo Toolbox behind OTB appication folder and OTB command line tools folder. Doubleclick beneath Value reveals an input line with a three-dot-icon, which lets you navigate to the folder.