My best SOT on the HTC was 7 hrs and 45 min which was looping videos overnight.
My best SOT on the Xiaomi was 10 hrs and 19 min under High Performance mode mostly videos, texting, and less than 2 hrs Wi-Fi. The Battery Saving could probably improve it to maybe an extra 40 min or 7-8% increase but I hate the 1 second lag when apps open. GSMArena was generally accurate that Wi-Fi is about 8 hrs and 10hrs video playback on the Mi 3.
Right now, I am at near 40% with close to 5 hrs SOT mostly Wi-Fi, downloading videos via TubeMate which kills battery faster, texting, and a few calls. Should finish around 7 hrs with 10% left like yesterday. The HTC is at 69% with near 2 hrs SOT while on airplane mode. I monitor my SOT like a hawk with either with Wakelock Detector or MIUI's battery usage menu. I don't use GSam Battery Monitor because the RAM is high and Du Battery offers a little more including providing SOT with similar RAM use. I just updated my PvZ game on both, so I probably will kill the battery faster later tonight.
OnePlus One from what I've been told really does have amazing battery life. Jon Rettinger even said so. PhoneArena ranked it only behind Sony Xperia Z2 back then. Best smartphone value of the year even with its flaws. Better than Nexus 5 or Moto X of last year. About 8-11 hrs SOT should be normal for you depending on your uses for each day.