Your phone isn't capable of charging any faster than it's built to charge. The Nexus 5 had very limited support for Quick Charge 1.0 due to its processor, although my understanding is that this functionality wasn't enabled for the 5. But the phones are designed to pull only as much power as they can safely pull. If it can't take it, it'll negotiate a lower rate with the charger.
That said... 20-30 minutes to charge completely doesn't sound possible. I'm not aware of a single phone that can do that. So something's a little bit odd there...
EDIT: Did some research and confirmed that this charge speed simply isn't possible. A few points:
- The most current I've ever seen a Nexus 5 pull from any charger (and I've tried a very wide range) was 1.2A. And boy was that fleeting. At that rate, it would charge fully in 2.3 hours, according to this battery charge time calculator. That's actually a little less than Anandtech found in their comparison chart found in the OnePlus One review, listing its charge time at 2.54 hours. That's more realistic, because as I said, getting a full 1.2A for more than a moment or two was quite rare, and phones draw less current the closer they are to full.
- According to that same review, the Nexus 6 is the second-fastest charging phone on the market (or was at that time), charging from 0% to 100% in 1.89 hours. Faster than the Nexus 5, but still a quarter of the speed you're saying you're getting on the Nexus 5. I just plugged my Nexus 6 into a QC2.0 charger, which I believe is still considered the fastest phone charging tech anyone's using, and right now it's pulling about 1.75A, which would be about 2.25 hours to fully charge. That said, it's nearly full, so that's when it slows down, which likely accounts for the difference between my figure and Anandtech's.
- Using that same battery charge calculator, to fully charge the Nexus 5 at your WORST-case scenario of a full 30 minutes, would require 5.52A of power. 8.29A for 20 minutes. Even the iPad, which has a massively larger battery and needs much more power to charge at a reasonable rate, uses only a 2.1A.
- The OnePlus One charger outputs only a maximum of 2A according to its own label. The best that charger could possibly do in an ideal world where it somehow can charge the Nexus 5 at full speed the ENTIRE time, is 1.38 hours.
Any way you slice it, it's not possible for a Nexus 5 to fully charge, with ANY charger, much less the OPO charger, in 20-30 minutes. If yours is informing you that it is, your Nexus 5 is faulty and displaying incorrect battery information. It's either displaying 100% when it's not full, or shutting itself off WAY before it's reached 0%. It's possible that this is an indication of the OPO charger somehow interacting badly with it, in which case I may rescind my initial advice that it can't hurt.