The 3T is ever so slightly pricier at £399, but then it does have double the storage and RAM - meanwhile the £369 Galaxy A5 2017 isn't as impressive on the specs side, but has the might of the Samsung brand behind it (Note 7 notwithstanding), and is arguably the more beautiful of the two handsets.
So - OnePlus 3T vs Samsung Galaxy A5 (2017), which one should you buy? Let's go head to head.
Galaxy A5 (2017):146x71x 7.9mm, 157g
OnePlus 3T: 153x 75x7.4mm, 158g
The OnePlus 3T is a little bigger, a little boxier and a little heavier, all of which makes the A5 comfier in the hand - but it's not a huge difference.
The matte aluminium finish of the 3T gives it a cool, textured feel, while the A5 is glossy but inevitably fingerprinty as hell as a result. Which look you prefer will come down to your preferred brand - if you generally like HTCs, you'll prefer the 3T. If you like Samsung devices, well, the A5 looks very much like its sibling the S7.
Samsung Galaxy A5 (2017)
Both phones have a rounded square lens in the centre back for the main camera, but the one on the OnePlus is considerably bigger and protrudes from the back, while the A5's sits flush. Branding is a little more subtle on the OnePlus, but mainly because people don't recognise the logo yet - and realistically, both will probably in a case anyway.
Colour-wise, you've got more choice with the Samsung - at least in terms of the bare chassis. That comes in a choice of Black Sky, Gold Sand, Blue Mist and Peach Cloud, while the 3T is available in just Gunmetal and Soft Gold. However, the official OnePlus cases in wood or carbon fibre finishes are really cool, and you'd be daft to finish ordering a 3T without adding one to your basket.
Then again, you'll always get a decent case selection with a Samsung - just in mainstream retailers, whereas for OnePlus you're mostly restricted to the manufacturer themselves.
In terms of buttons, the OnePlus 3T has a physical Do Not Disturb switch, which the Galaxy A5 doesn't offer. Both phones have capacitive buttons either side of the home key, but the A5's are the wrong way round (compared with most Androids, at least). Back is on the right and Apps is to the left. This can't be changed, either. The OnePlus is much more customisable, so although the Back and Apps buttons are in the standard positions, it's easy to swap them in the software if you prefer the Samsung way round.
The home keys on the two phones are similar, but the big difference is that Samsung's presses in and OnePlus's doesn't. They're both similar-sized ovals below the display on the front of the phone, and both have the fingerprint sensor built-in - it just depends if you prefer the HTC-style unpressable pad or the iPhone-style clicky button.
The 3T's speaker is on the bottom left edge, while the Samsung's is on the top right (unusually). Neither are especially great, sound-wise, though the OnePlus is easier to muffle while you're holding the phone in hand.
Finally, you'll find a USB C charging port on the centre bottom of both handsets, with - yep - a 3.5mm headphone jack to the right of both. Nothing to judge it on here.
Winner: Draw. The Samsung looks more expensive and has more colour options, but oh lord the fingerprints. This is going to come down to personal style, and whether you're getting a case.
Galaxy A5 (2017): 5.2i, full HD (1920x1080), 424ppi
OnePlus 3T: 5.5in full HD (1920x1080), 401ppi
Oooh, tricky. Do you prefer a bigger screen, or a better screen? Both displays are full HD AMOLEDs, but due to its smaller size the A5's has a higher pixel density. Realistically, this isn't going to make a huge difference, especially if you're not using the phone for VR (and if you were hoping to use the A5 with the Gear VR, you're out of luck - it's not supported).
Samsung Galaxy A5 (2017)
The A5 does include Samsung's always-on display, though, making use of that Super AMOLED screen to display the time, calendar and so on without having to power up the screen. If you're a compulsive time-checker, you might love this feature.
Both panels are coated with extra-tough Gorilla Glass 4, and both are bright, colourful, clear and responsive. You won't be disappointed with either one.
Winner: Draw. OnePlus offers more screen space but Samsung packs in more pixels per inch, and Samsung displays are among the best in the industry. So which one you should go for will come down to this: do you want the slightly-bigger screen, or the slightly-better screen?