There are many Oracle components that run a web service, so it's not clear which you are referring to.
For example, the web site port for standalone OC4J is configured in the j2ee/home/config/default-web-site.xml file:
<web-site xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:noNamespaceSchemaLocation="http://xmlns.oracle.com/oracleas/schema/web-site-10_0.xsd"
port="8888" display-name="OC4J 10g (10.1.3) Default Web Site"
schema-major-version="10" schema-minor-version="0" >