As you know we have two products related to Share Point technology, WSS (Windows SharePoint Services) & OSS (Office SharePoint Server previously named SharePoint Portal Server or SPS).
OSS implemented on top of WSS and adds a huge set of features and facilities to the great foundation of WSS, but OSS uses WSS as its core technology.
Also we know that WSS is a free license software as a service by Windows Server 2003, and we can implementing a large scale of collaborations and business solutions based on SQL Server, Active Directory, WSS & Visual Studio 2005.
All these tools help us implementing solutions and running enterprise projects so fast and with a minimum budget!
As a solution provider of these technologies with a 5 enterprise and operational projects based on WSS (can see details on about), I’m an adviser to my colleagues: only use OSS when you really need at least 30% of its features.
I think we can use WSS for free to implement a huge set of solutions and applications, and we can implement some features of OSS that you really needed on it (especially on the WSS3), remember that OSS implemented on top of WSS, and you can do same, but we must decide a trade off by time and budget, as a result: if you need just some small features of OSS, you can use WSS and add that features on top of it instead of using OSS.
Briefly , use of OSS have some loads about money, administration, maintenance and most important: training of users to use, and I advise choose it only on the really enterprise organizations and only when you need at least 30% of its facilities.
In other projects you can use WSS for free as a slim, really functional & stable platform, and it has a wide object model, letting us adding almost any features we need!