Beside making sure the Oracle 64-bit client is also installed on the machine where both ArcGIS Desktop and ArcGIS Server is installed, please also set the PATH variable to read the 64-bit client before the 32-bit client. Please refer to the documentation: http://resourcesbeta.arcgis.com/en/help/main/10.1/index.html#//002n00000038000000
... View more