Middle tier servers technology is part of DBA job, depending upon the size of the shop you will be working for. In the current e-biz market you are not just backend guy anymore like in early 90's. You need to have knowledge of OAS and Microsoft IIS and other products to have better integration of the database and front end user interface. Your data architect/analyst will be discussing with you what business processes they will be implementing on middle tiers and how you will be supporting those processes from database server. The knowledge is mandatory if you are consultant or your shop doesn't have clearly defined job roles.
I would say knowing OAS is another addition to your resume. The web servers provided by oracle are capable of embading the PLSQL modules into them. That is one of the advantages of using the oracle's application server. Knowing this would help you to intrergrate the oracle products to the web. But it is not againg necessassarily be true. If you are just planning to know about this product, then I would suggest that start with iAS.
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...
If I was you, definitely forget about OAS unless your shop is using it and they need you to administer it. OAS is not bad but compare with others... The advantage is its tight integration with Oracle 8i. iAS is different story, they totally re-coded the whole application server and uses Apache as web server becoming more scalable. OAS and IAS has nothing to do with each other btw