More information is required.
What is the OPA service being used (Assess? Connector Framework?)
If the XML output from OPA is correct, and the target system is reporting erroneous values after saving..
- What is the target system?
- Is the erroneous nature of the data confined to one data type - such as dates or numbers?
For example, I've seen issues with Siebel for example misinterpreting dates due to region / time zone differences not being handled properly in the Workflow / XSLT, which resulted in certain dates appearing one day later in Siebel.