This question leads to the ever popular answer: it depends.
There are more than one Windows Defragmentation tool. There are differences in versions of Oracle. There are complications if you are using third party products like VM environments, SAN management, backup tools, etc.
Another question that I'd ask would be "how much will defragmentation help database access?". If your sysadmin doesn't have a good solid answer, then I'd resist doing a defrag just because it sounds like a good idea... If there isn't a clear benefit, why risk it?
The short answer is that when Oracle and the associated tools are down and not running in any way it is nearly always safe to defragment a drive. The more programs that might access the disk while the defragmentation is occurring, the higher the risk of database corruption.
If the data is redundant or isn't especially valuable such as a staging database for a data warehouse or clickstream data which is rarely if ever needed, then you can probably get by defragging whenever you choose. If downtime is hard to schedule, you at least need to make a backup that you can take offline just before you defrag. If the data is really valuable, then I'd take a backup and then quiesce everything that I could that might access the disk before running the defrag.
In theory, theory and practice are identical. In practice, theory and practice are unrelated.