ul.o_checklist > li {
    list-style: none;
    position: relative;
    margin-left: 20px;
}
ul.o_checklist > li:not(.oe-nested):before {
    content: '';
    position: absolute;
    left: -20px;
    display: block;
    height: 14px;
    width: 14px;
    top: 1px;
    border: 1px solid;
    cursor: pointer;
}
ul.o_checklist > li.o_checked:after {
    content: "✓";
    transition: opacity .5s;
    position: absolute;
    left: -18px;
    top: -1px;
    opacity: 1;
}