\documentclass{article}
\usepackage{fontspec}
\setmainfont{OpenSans-Regular.ttf}[
LetterSpace = 2,
FontFace = {lt}{n}{OpenSans-Light.ttf},
FontFace = {sb}{n}{OpenSans-Semibold.ttf},
FontFace = {bf}{n}{OpenSans-Bold.ttf},
FontFace = {bx}{n}{OpenSans-Bold.ttf},
FontFace = {xb}{n}{OpenSans-ExtraBold.ttf},
FontFace = {lt}{it}{OpenSans-LightItalic.ttf},
FontFace = {md}{it}{OpenSans-Italic.ttf},
FontFace = {sb}{it}{OpenSans-SemiboldItalic.ttf},
FontFace = {bf}{it}{OpenSans-BoldItalic.ttf},