I'd like to move the secondary sidebar from Oxygen entirely, so my images and content can extend across two column layout not a third column.

To hide the secondary sidebar ( the one on the left ) on the Oxygen theme showcase page template, add this to your Appearance → Custom Design → CSS editor:

.page-template-showcase-php #secondary {
	display: none;
}

.page-template-showcase-php #content-wrap {
	width: 100%;
}

.page-template-showcase-php .wp-post-image {
	width: 588px;
}

To hide the secondary sidebar on posts and pages in the Oxygen theme, add this:

#secondary {
	display: none;
}

#content {
	margin-left: 0;
}