"What are the two things you can't live without?" A teaser so vague OnePlus had to actually clarify that it's a teaser for "something we've been cooking up." It doesn’t help that the picture used in the teaser is of the OnePlus One, the company's original product.
Given that it's not either of OP's current devices – the 2 or the X – it could be some good news for owners of the original.
It was updated to Cyanogen OS 12.1 recently (that's Android 5.1) and got 6.0 Marshmallow only as an unofficial CyanogenMod 13 port. Is this it? An update? The OnePlus 2 has been confirmed for a Marshmallow update, so maybe. There's OnePlus' own Oxygen OS.
Until OnePlus provides a less vague teaser, enjoy the guesses of Twitter followers (hit the source link).