OnePlus started selling smartphones a few years ago with an invite system, which allowed it to carefully manage its supply chain. It was also hugely annoying for people interested in actually buying phones from OnePlus. The OnePlus 3 was launched sans invite, and it's a much more attractive option because of it. However, OP says it can't keep up with demand, so it's pausing sales in Europe and Hong Kong for a month.

Since launch, the shipping timeline for the OnePlus 3 has been getting distressingly long. Right now it's around two weeks after purchase for the US. OP says sales in mainland Europe have been substantially higher than expected, so the company is going to stop selling the phone from August 9th to September 12th in some countries to allow stock to replenish (technically a little more than a month). The affected countries are as follows.

Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Estonia, France, Greece, Hungary, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, and Hong Kong.

You've got a few days to get an order in if you're in one of the affected countries. You'll have to wait a couple weeks for shipping, but you'll still get your phone faster than waiting until sales are on again. During the sales pause, there will be a notification option to let people know when the phone is available for purchase again. This is a pretty drastic step to take—OnePlus must not have anticipated how difficult it can be to launch a phone without the invite system crutch.