Oxygen has the same ditaval functionality built into its interface, so you don't need to create a custom ditaval file in order to have the rules reflected in your output. You may be pushing your content out to another system, however (such as Fluid Topics or Suiteshare), and in that case, you'll need to create ditaval files. Having the ditaval files available in your dita files will instruct the system on how to publish your content.

The ditaval file consists of the following:
    <prop action="exclude" att="platform" val="mac" />
    <prop action="exclude" att="platform" val="linux" />
    <prop action="exclude" att="product" val="acme"/>

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You specify the action (almost always exclude), the attribute (att), and the value (val). In this ditaval file, any properties that have platform="mac" or platform="linux" or product="acme" will be excluded from the output. Any elements with those attribute values will be excluded.

(You can change the default action to include and run more sophisticated logic in your ditaval file if you so choose. But by default everything is included. The basic idea is to specify what you want excluded.)