My OnePlus 3 dash charger is rated 200-240V. I will be travelling to a 100V country and wondering
(a) If my dash charger can be plugged into a 100V socket without damaging it or the phone and
(b) Whether it will deliver Dash charging speeds.
Simple answer is No, it will not work the way it is supposed to, if it works
Any charger for that matter any electrical appliance can only deliver what is expected of it when it operates under the conditions it is designed for.
That is what standards are meant for and that is what makes it mandatory to be adhered to
Your charger work may work erratically charging normally but definitely not dash charge, while it may not damage the phone. Quite likely the charger won't even fit into the wall adapter there !
Even within standards, compatibility and ability to fast or dash charge is tricky at times as you can see from the table for one plus 2 from OEM site given the nature of dash or fast charging
So, how do I charge?: Use a power bank to charge your mobile, Googling should help you find one that is rated for 100 V. Quick search throws up one such power bank. It may be easier to request somebody in the place you are visiting to buy it for you