First you would need to root your phone without installing a custom recovery:
For the One Plus One, download SuperSU flashable package from http://download.chainfire.eu/396/SuperSU/
Now, download TWRP recovery from here: http://techerrata.com/browse/twrp2/bacon
The usual method of rooting is to:
1. Unlock bootloader.
Press Vol +, Power button at the same time after switching off phone. Then release Power button keeping Vol+ button pressed. You will be landed in fastboot mode.
Enter the following:
Note: Unlocking bootloader automatically wipes everything.
fastboot oem unlock
Now bootloader is unlocked and you will be booted to a clean fresh ROM.
Next, enable USB debugging.
Enable Developer options by pressing on the Android version number 10 times.
Now go to fastboot mode again to boot a custom recovery.
After downloading the TWRP recovery, you can boot off it without installing the recovery by running:
fastboot boot openrecovery-twrp-220.127.116.11-bacon.img
Once TWRP opens up (give it 10-15 secs), choose to reboot to system. TWRP has an automatic system to check whether the phone is rooted, and to install SuperSU properly, rooting your phone in the process. If that doesnt happen, you can repeat the process, booting to TWRP, and then flashing the zip file of SuperSU you downloaded earlier.
Kudos..you have successfully rooted your phone, preserving stock recovery. The advantage is that you will continue to receive OTA updates while having root too. Usually custom recoveries are unable to flash OTAs.
If you wish to have TWRP as your main recovery, just flash it from fastboot mode:
fastboot flash recovery openrecovery-twrp-18.104.22.168-bacon.img