i still use onestep all the time, but as a cleaner rather than a sanitizer. i soak in onestep for an hour (or more if i think it needs it) and then rinse and give everything a quick spray down with star san. oxyclean works as a cleaner as well (it's also NOT a sanitizer) but i feel like onestep leaves less residue when soaked for extended periods.
to answer your original question, you can store star san in your primary (and many do). star san is good as long as the ph stays below 3.5 which can be quite a long time if you make it with distilled water.