Looking to display the mobile hamburger menu icon all the time in your Oxygen site?

Just add this CSS:

.oxy-menu-toggle {
    display: block;
}

.oxy-menu-toggle+div {
    display: none;
}

.oxy-nav-menu-open .oxy-menu-toggle+div {
    display: block;
}

You might want to select the Menu element and go to Mobile Responsive > Menu Styles to set a Link Hover Color in case you find that menu items in the overlay are not visible (white on white) when hovered.