The best replacement for open sans is source sans.

The best alternatives to open sans are rated as following:
★★★★★  source sans
★★★★★  source sans pro
★★★★  segoe ui
★★★★  lato
★★★★  droid sans
★★★  roboto
★★★  calibri
★★★  pt sans
★★★  myriad pro
★★  clear sans
★★  noto sans
★  proxima nova
★  helvetica neue
★  verdana
  segoe

Your alternative for open sans is not listed? Suggest!
Speaking about open sans, one probably shouldn't forget about serif.

Other search terms related to open sans and their ratings are:
★★★★★ serif
You have in mind a companion for open sans and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of open sans in all years. Click on graph for more.
open sans
source sans
source sans pro