I'm not much of a Windows guy, but it looks to me that perhaps the problem is that Oracle.dll needs to be able to find additional libraries, most likely the Oracle client libraries. Someone better at windows could probably tell you how to figure out whether Oracle.dll has all its dependancies met. A quick google search found me Dependancy Walker, but I've certainly never used it, and there may be a better way.

Did you install DBD-Oracle from source, or CPAN? Is it something packaged with your perl distribution? Do you have the Oracle client libraries installed and working?