As a way of introduction, wikipedia generally defines these terms as:
SOA - First proposed by Gartner analysts, SOA expresses a perspective of software architecture that defines the use of services to support the requirements of software users. In an SOA environment, resources on a network are made available as independent services that can be accessed without knowledge of their underlying platform implementation.
RIA - Though the concept had been around for a number of years, the term RIA was introduced in a Macromedia whitepaper in March 2002. RIA applications are Web-based applications that run in a Web browser and do not require software installation, but still have the features and functionality of traditional desktop applications.
OSS - Open source software is computer software whose source code is available under a copyright license that permits users to study, change and improve the software, and to redistribute it in modified or unmodified form.
