Between OnePlus' relation with Oppo, the documented specifications, etc. - Pretty much every difference in the defconfigs/source for the two devices corresponds to the few documented differences in specs between the two devices.
The OnePlus One is a Find 7a with the following tweaks:
1) Nonremovable battery (kernel change: battery authentication disabled)
2) A PN65T instead of PN544 (kernel change: PN65 support in the NFC driver. The PN65 is pointless now that Google has sunsetted hardware SE support)
3) RGB LED instead of the blue-only Skyline (kernel change: different LED driver)
4) Left/right "mirror" reversal. (power button on right of OnePlus, left for F7a, etc.) Kernel change: none
5) MSM8974AC instead of MSM8974AB (kernel change: none, handled automatically at runtime)