OAS simplified the ability to generate html based on data from an Oracle backend. Although not as popular versus other webserver (see http://www.netcraft.com for surveys) it provides the ability for code to be generated from servlets,plsql,c, and others...

Unforntunately, the overhead associated with OAS limited the scability and overall performance compared to apache based webservers.

Oracle, and other large vendors, (I believe IBM and BEA) have license/adopted Apache as the http process of choice.

I've migrated several oas installs to IAS pretty easily and have found the performance gains fantastic. I've also added Webcache and database cache to a couple of installs with good results...