No, this is not another rant about just anyone who doesn't know how to read manuals. It's about me. And I don't know how to read manuals.
After downloading Oracle 10g for Mac OS X, I spent some time getting it installed. After a false start (where I created the dba group with GUID 201 - the same as the Guest account on Mac OS X), I managed to get it up and running. There's an excellent guide here, which walks you through just what you need to do and little more: How to install Oracle Database 10g on Mac OS X Intel
Once I could SQL*Plus into the database, I downloaded the Companion CD, and started to install that, thinking that the Oracle HTTP Server would be a part of it.
To read the entire article at its source, please refer to RTFM