ORM (esp. NHibernate) performance for complex queries

Question

My company is in the process of rewriting an existing application from scratch. This application, among other tasks, performs complex SQL queries against order and invoice data

Answer 1

If you are looking for ORM with comprehensive and efficient LINQ translator, I recommend you to try Entity Framework or DataObjects.Net. I'm not sure about Oracle Lite support. NHibernate's LINQ translator is not fully finished yet.

Anyway the best thing you can do now is to download leading ORM tools and test them on tricky queries with grouping, joining, subqueries etc.