By default if 'Recent Posts Widget With Thumbnails' is use with Oxygen Builder, you need to insert below CSS line

.rpwwt-widget ul {
    list-style: outside none none;
    margin-left: 0;
    margin-right: 0;
    padding-left: 0;
    padding-right: 0;
}
.rpwwt-widget ul li {
    overflow: hidden;
    margin: 0 0 1.5em;
}
.rpwwt-widget ul li img {
    width: 75px;
    height: 75px;
}
.rpwwt-widget ul li img {
    display: inline;
    float: left;
    margin: .3em .75em .75em 0;
}
.rpwwt-post-title {
    font-size: 16px;
}
.rpwwt-post-date	{
		font-size:14px!important;
}