It's just too thin. There's a reason thin fonts on mobile OSes are used only for big text like date and time. Thin fonts are difficult to read, Open Sans particularly so.
Hmmm.. That's strange considering Steve Matteson (the type designer)[1] optimized the typeface for both mobile and web use. Perhaps the mobile browser you're using is rending it quite differently.
I'd love to know what browser / os you're using. I'm running a galaxy s3 in 'tablet mode' (lcd density set to 160 from 320) and the text renders nicely.