The Marshmallow update for the OnePlus One is not only in the works, but is in its final testing stage. OnePlus co-founder took it to Twitter to share the good news. Marshmallow for the One is in the final testing stage. Can't give ETA as it depends on other parties, but should be very soon. Carl Pei (@getpeid) April 6, 2016 Android 6.0 was promised to One users back in November last year, with a planned release date in Q1 2016, which obviously didn't happen. At this point it's still not entirely clear when the update will actually make it to the masses, as it's dependent on...
Source: http://www.gsmarena.com/oneplus_ones_marshmallow_update_in_final_testing-news-17652.php