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:
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:
This even works with the standalone installers.
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.