I would assume (perhaps wrongly?) he's referring to mixins, which allow you to easily re-use chunks of declarations (or objects). The pure OOCSS alternative to this is to separate your classes more.

Quick example, OOCSS way of making the notion of 'fancifying a thing', is to make a separate class to represent 'fanciness'.

  .fancy { color: hotpink; font-style: italic }
  .thing { width: 100px; height: 40px }

Then in your HTML apply both classes to make a fancy thing: <div class="thing fancy">ta-da</div>

Whereas in SASS you could do that with a mixin instead:

  @mixin fancy { color: hotpink; font-style: italic }
  .fancy-thing { width: 100px; height: 40px; @include fancy }

And in your HTML, you then apply a single class: <div class="fancy-thing">bam</div>

There are advantages of both, and there are clearly variations you could make on the SASS version (i.e. you could make a separate thing mixin too, then mix both into a single fancy-thing class - making the thing concept as reusable as in the CSS version).

Personally I like the level of understanding that doing it the CSS way brings (note this is also an example of the separating 'skin' from 'structure' idea, which I think is very valuable). You could argue that it could lead to class-itis though. Swings and roundabouts.