$('.btn').hover(
function () {
    var srcLength = $(this).attr('src').length;
    var baseName = $(this).attr('src').substr(0, srcLength - 4);
    $(this).attr('src', baseName + '_o' +  $(this).attr('src').substr(srcLength - 4, 4));
},
function () {
    var srcLength = $(this).attr('src').length;
    var baseName = $(this).attr('src').substr(0, srcLength - 6);
    $(this).attr('src', baseName + $(this).attr('src').substr(srcLength - 4, 4));
}
)