By today's standards, we'd call the OnePlus 2 a phablet (if just barely) while the Galaxy S6 is more of a "regular" flagship size. The OnePlus phone is 6 percent taller and 6 percent wider, while Samsung's phone is 31 percent thinner.
Much like an iPhone, the Galaxy S6 feels incredibly light in hand: it's 21 percent lighter than the OnePlus 2.
The OnePlus 2 ships with a sandstone backing (probably a thin layer on top of plastic), but you can also order replacement backs in natural and kevlar materials.
The Galaxy S6's back is made of Gorilla Glass 4. Its design is among the best (alongside the iPhone 6 and HTC One M9), and if you want an even better-looking version, you can opt for the curved Galaxy S6 edge.
Both handsets have metallic frames.
Though the OnePlus 2's back color may change if you order any of those replacement backs, it ships with that black sandstone one.
The OnePlus 2 has a 16 percent bigger screen.
The Galaxy S6 has one of the best – probably the best – displays of any smartphone you can buy today, squeezing in an insane 577 pixels per inch.
The Galaxy S6's AMOLED panel has rich colors, great contrast and deep blacks.
We haven't yet tested it, but the OnePlus 2 has a touch-based fingerprint sensor that should be similar to the one in the GS6.
The OnePlus 2 has a higher-capacity battery, but we don't yet know what that will translate to, as far as battery life goes.
The Galaxy S6 scored well in our battery benchmark: only dropping 11 percent per hour when streaming video over Wi-Fi (with brightness set to 75 percent).
You can't swap batteries on either of these puppies.
The GS6 has a fast-charging feature that lets you quickly get a nearly-dead battery back to a respectable state.
The GS6 has built-in wireless charging, compatible with both the Qi and PMA standards.
The Galaxy S6's rear camera has the higher resolution. That doesn't directly translate into better results, but its shooter does happen to be one of the best today.
Camera aperture (rear)
The GS6's rear camera wins by a pinch on aperture.
Both handsets' rear cameras have Optical Image Stabilization, to help minimize the effects of shaky hands.
The GS6's entry-level tier has double the storage of the OnePlus 2's, and Samsung also offers a 128 GB option that OnePlus doesn't.
Before this year, Samsung flagships and microSD card slots went hand in hand, but that went the way of the dodo with the GS6.
Both phones have speedy octa-core, 64-bit processors.
The Snapdragon 810 had some (noticeable, but not dire) heat output issues in the One M9 and LG G Flex 2, so we'll have to see whether the OnePlus 2 fares any better.
The 16 GB OnePlus 2 gives you 3 GB of RAM. That jumps up to 4 GB, though, in the 64 GB model.
After years of showing up in Android flagships without having a clear purpose, NFC is finally finding a mission in life with the emergence of mobile payments. That makes it all the stranger that OnePlus left NFC out of its new flagship.
That means Android Pay will be a no-go on the OnePlus 2 when the next version of Google's OS launches later this year.
Heart rate sensor
The Galaxy S6 has a heart rate sensor hanging out beneath its camera, on its backside.
The OnePlus 2 has a physical notification toggle on its side, that lets you swap from "All" to "Priority" to "None" sound profiles without touching its screen.
Both phones run Android Lollipop. The OnePlus 2 runs a mostly stock version of Android, keeping Google's visual theme and features whle only adding a few extra goodies (like a dark mode and the choice between onscreen or capacitive navigation buttons).
Samsung's TouchWiz UI is more of a visual departure from stock Android, but its glitz and feature list are much subtler than they were in previous Samsung flagships.
Some people will be able to buy the OnePlus 2, starting on August 11.
Buy without invite
But you will need to sign up and wait who-knows-how-long for an invite before you can order the OnePlus 2.
Starting price (full retail)
We've seen some recent deals on the Galaxy S6 (T-Mobile currently offers it for US$100 off), but this $650-ish full retail price is still what we see in most places. Of course all the carriers offer installment plans, and several use the two-year contract model, to lower your upfront cost (that usually falls between $200 and $0 down).
The OnePlus 2, if you can get an invite, offers outstanding specs for its price point. Though high-end specs alone don't make a phone, they're a great starting point.
For more, you can check out Gizmag's full review of the Galaxy S6.