


In order for the images to auto fit to slider containers use width 100% on . Check CSS below
$(document).ready(function() {
  $("#owl-demo").owlCarousel({
      navigation : true, // Show next and prev buttons
      slideSpeed : 300,
      paginationSpeed : 400,
      singleItem:true
      // "singleItem:true" is a shortcut for:
      // items : 1, 
      // itemsDesktop : false,
      // itemsDesktopSmall : false,
      // itemsTablet: false,
      // itemsMobile : false
  });
});
  
              <div id="owl-demo" class="owl-carousel owl-theme"> <div class="item"><img src="assets/fullimage1.jpg" alt="The Last of us"></div> <div class="item"><img src="assets/fullimage2.jpg" alt="GTA V"></div> <div class="item"><img src="assets/fullimage3.jpg" alt="Mirror Edge"></div> </div>
#owl-demo .item img{
    display: block;
    width: 100%;
    height: auto;
}