Why do I get the following error when executing a package under SQL Server Agent Job or DTEXEC?

Why do I get the following error when executing a package under SQL Server Agent Job or DTEXEC?

System.IO.FileNotFoundException: Could not load file or assembly 'Oracle.DataAccess, Version=2.111.6.20, Culture=neutral, PublicKeyToken=89b483f429c47342' or one of its dependencies. The system cannot find the file specified.

We have seen this error when using Oracle Data Provider for .NET 11.1.0.6.20 on 64bit OS, executing in 64bit mode. The provider doesn't support 64bit execution. Please execute your Oracle bulk-load packages in 32bit mode.