id为定向选择依据，class类选择器为辅助，创建派生选择器即可定向控制

<div class="modal fade" id="O_Modal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel" aria-hidden="true">
    <div class="modal-dialog">
        <div class="modal-content">
            <div class="modal-header">
                <button type="button" class="close" data-dismiss="modal" aria-hidden="true">
                    &times;
                </button>
                <h4 class="modal-title" style="text-align: center;">
                    修改岗位
                </h4>
            </div>
            <div id="toolbar">
                <button type="button" onclick="O_select()" class="btn btn-primary btn-sm">提交</button>
            </div>
            <div class="modal-body">
                <table class="table" id="O_Table"></table>
            </div>
        </div>
    </div>
</div>

<style type="text/css">
    #O_Modal .fixed-table-body {
        height: 275px !important;
    }
</style>