<!-- no HEAD -->
<link href='http://fonts.googleapis.com/css?family=Open+Sans:400,300,700''>http://fonts.googleapis.com/css?family=Open+Sans:400,300,700' rel='stylesheet' type='text/css'>
Ou acessando a url contida no atributo href obter a diretiva fonte-face:
@font-face {
font-family: 'Open Sans';
font-style: normal;
font-weight: 300;
src: local('Open Sans Light'), local('OpenSans-Light'), url(http://fonts.gstatic.com/s/opensans/v13/DXI1ORHCpsQm3Vp6mXoaTa-j2U0lmluP9RWlSytm3ho.woff2) format('woff2');
unicode-range: U+0460-052F, U+20B4, U+2DE0-2DFF, U+A640-A69F;
}
Também usando o import gerado pelo próprio google fonts:
@import url(http://fonts.googleapis.com/css?family=Open+Sans:400,300,700);
E depois aplicar no CSS através da propriedade font-family:
font-family: 'Open Sans', sans-serif;