Using @extend in a @mixin for OOCSS Structre - http://sassmeister.com/gist/d4b500eb6b230b3a845a

SassMeister-input-HTML.html

<h1 class="header__logo">Header Logo</h1>
<span class="footer__logo">Footer Logo</span>

SassMeister-input.scss

//
// Article Info(Fun Part):
//
@mixin person($who) {
  #{$who}-name: "Parhum Khoshbakht";
  #{$who}-url: "http://parhum.net";
  #{$who}-twitter: "@parhumm";
}
%article__info {
  article-title: "Using @extend in a @mixin for OOCSS Structre";
  article-url: "http://parhum.net/blog/ui/oocss-styleguide-with-sass/";
  
  @include person(author);
}

 //
 // Codes Here:
 //
 %helper--text-hidden {
  text-indent: -9999px;
  line-height: 1px;
  font-size: 1px;
  overflow: hidden;
  display:  block;
}
 
@mixin text-hidden($width, $height) {
  @extend %helper--text-hidden;
  
  width: $width;
  height: $height;
}
 
 
.header__logo {
  @include text-hidden(150px, 113px);
  
  float: left;
  background: url('http://parhum.net/wp-content/uploads/2014/08/sass-white-logo.png') no-repeat 0 0;
}
.footer__logo {
  @include text-hidden(75px, 56px);
  
  margin: 0 auto;
  background: url('http://parhum.net/wp-content/uploads/2014/08/sass-black-logo.png') no-repeat 0 0;
}

SassMeister-output.css

.header__logo, .footer__logo {
  text-indent: -9999px;
  line-height: 1px;
  font-size: 1px;
  overflow: hidden;
  display: block;
}

.header__logo {
  width: 150px;
  height: 113px;
  float: left;
  background: url("http://parhum.net/wp-content/uploads/2014/08/sass-white-logo.png") no-repeat 0 0;
}

.footer__logo {
  width: 75px;
  height: 56px;
  margin: 0 auto;
  background: url("http://parhum.net/wp-content/uploads/2014/08/sass-black-logo.png") no-repeat 0 0;
}