自适应高度

$(function(){
    $('#owl-demo').owlCarousel({
        items: 1,
        autoPlay: 3000,
        autoHeight: true,
        transitionStyle: 'fade'
    });
});

<div id="owl-demo" class="owl-carousel">
    <a class="item"><img src="img/900x300.png" alt=""></a>
    <a class="item"><img src="img/900x500.png" alt=""></a>
    <a class="item"><img src="img/900x200.png" alt=""></a>
    <a class="item"><img src="img/900x400.png" alt=""></a>
</div>

#owl-demo {
    width: 900px;
    margin-left: auto;
    margin-right: auto;
}
#owl-demo .item {
    display: block;
}
#owl-demo img {
    display: block;
    width: 100%;
}