Sadly it looks like OnePlus is moving away from Cyanogenmod (search Google News for `oneplus cyanogen`). Also the OnePlus One never had the fully open source version of Cyanogenmod 11. It uses a version called 11s (https://cyngn.com/products/oneplusone/) with many proprietary features.

However, they will surely continue making phones at that price on which the pure version of Cyanogenmod can be installed.

Xiaomi seems very promising since they also make phones at that price point with similar specs, and Cyanogenmod ROMs are in development. I know it might be hard to get them in the US, but in Europe they can even be ordered from Amazon.