While OnePlus (i've owned all OnePlus phones since OP1) is a great example however there is a common misconception that it was "new" at that time. The OnePlus 1 was almost identical to the Oppo Find 7A in specs, price, etc [1]. Also not commonly known, OnePlus was started by an ex-Oppo executive and is actually (majority) owned by Oppo Electronics which is the parent company of Oppo Mobile [2].

Also while price was definitely a huge factor for it's popularity, the main reason IMHO was because it was the first phone that officially supported a popular AOSP at that time - CyanogenMod (now LineageOS [3]) and it even had the logo branded on the back. Almost everyone I knew with a OP1 phone bought it simply for cyanogenmod and how fast/smooth it ran it with no issues. Remember that when OnePlus 1 was first announced in 2014, android phones were well known to be filled with vendor bloatware (Samsung Galaxy S3,S4 anyone?), expensive, became really slow over time and for those of us that wanted custom ROMs, it was relatively painful to flash/root a stable AOSP on any device. OnePlus 1 was the first phone that I had absolutely no issues with having CyanogenMod on it such that I even turned on nightly updates knowing full well that it was tested at least on a OnePlus device.

So yea, I think OnePlus became popular simply because of price and their official support for CyanogenMod which meant a phone with a fast & responsive running android and most importantly, no bloatware.

[1] https://www.gsmarena.com/compare.php3?idPhone1=6214&idPhone2...

[2] https://www.gizchina.com/2014/04/28/oneplus-responds-oppo-co...

[3] https://www.androidauthority.com/cyanogenmod-lineageos-65481...