Q - What is the difference between EUNA and OAMP? Do I need both?
A - The two are definitely designed to work together as bookends on a pedal rig, although one of the reasons I put them in separate boxes is so you could choose for yourself. EUNA is tweaked for the guitar's input directly, and tweaked to drive a chain of pedals after it. OAMP is tweaked to take the output from your last pedal and drive into the amp, or drive a power amp directly, or a direct-input of some kind. The input impedance of OAMP is not as high as EUNA, because we're assuming it's later in the chain. It's high enough to still sound good with a guitar direct in, and definitely tweaked to be hit with a fuzz, but I was able to do some different things in the circuit because I could presume a lower source impedance than with EUNA. Magnetic pickups want almost the highest input impedance you can give them, but that's not always ideal for other pedals. There are some noise issues, tone issues, basically it begins to explain why too many buffers sounds bad even though each one individually sounds fine. Bridging Impedance is something we do for convenience in the modern world, but some would say Matching Impedance sounds better. It's just hard to implement with so many builders and designers in the world, and so many users with their own unique applications. One of the reasons I tune my devices to specific applications is to try to cheat that a bit, basically still build Bridging Impedance devices that are flexible while getting some of the tonal and drive benefits of more closely matched impedances like the fabled gear of old. Some of that is provable, some is superstition, you can make your own call if it works for you.
Their filters are also slightly different, EUNA can add lows, add brightness and turn up the harmonics. OAMP's bright filter is shaped a little differently, and it also has the Dark filter which EUNA does not. The Dark filter rolls off some top, so if you wanted to drive your pedalboard with some extra top end, shape the signal a bit to hit drive pedals, but not have the overall tone be too bright, you can just click that in and go with it. Internally, a lot of good-sounding drive circuits use pre-emphasis/de-emphasis, so EUNA and OAMP combined can let you do that across your whole board globally, and even control it a bit.
There is some overlap is application though. Even though it wasn't designed specifically for it, you could use OAMP in place of EUNA as an input driver. One reason some players don't like to have an input buffer is because they don't want the top end back - they like a little top-end roll-off right up top and use it to get their sound. When EUNA was on That Pedal Show, Mick didn't love it because it gave back a bunch of top end that he was used to being rolled off - his whole rig was tweaked for it and his tone is awesome. Since OAMP has a slightly lower input impedance and a tiny bit more capacitive loading, plugging a guitar directly in wouldn't get back quite as much top end as EUNA, although depending on the guitar it may be very minor. Similarly, EUNA has great headroom and can work as an output driver from your board, there's just not a lot of gain-in-hand to tweak the drive, but it will push the signal down the wire equally well. Both are high-headroom devices as far as typical guitar pedals go, both have been tested extensively down to a 600 ohm load.
In summary, they are specialized but flexible. Hopefully one or the other or both solves some problems for you and helps you get closer to your ideal tone!