Feel free to correct but I hope this helps. I also tried to categorize them for most recent two generations, to provide more "apple to apple" comparison. some are very obvious and some also have my personal opinions, so do take it grain of salt. op has different dial colors, but black dial used for more "fair comparison".
delta bewteen 39mm (214270) explorer vs 39mm OP (114300) black dial
1. mercedes hand (214270) vs baton hand (114300). from here, left will be the explorer, right will be OP 39
2. 214270 clasp has easy link and fliplock, as well as "extruded rolex crown". OP39 clasp lacks easy link (unless AD secretly installed one for you), has no fliplock, and has "laser etched rolex crown". if you have both, do try clasp swapping and share the results here!
3. explorer 369 dial is matte black,with a small white painted crown logo, with all indicies lumining, except, for mark 1 only, 369. op39 dial is more like a sunray black and has lumes througout the indices, except the 12'o clock crown
4. the explorer bezel is more flat and slanted, while op bezel is more domed.
I personally was in market for both and chose the mk1 214270 explorer. the clasp and the explorer bezel were the dealmakers. OP looked a bit stumpy with its fatter bezel, and looked cheaper, because of the bezel. dimensions (lug to lug, lug mm, case mm, thickness, etc), for these two watches, otherwise, i believe are idnetical.
here is the delta between 36mm (124270) explorer vs 36mm OP (126000) black dial. for this comparison, I am not as confident as I was on 39mm, so ppl please help me with second set of eyes.
1. mercedes hand (124270) vs baton hand (126000). from here, left will be the explorer, right will be OP 36
2. 124270 clasp has easy link and fliplock, as well as "extruded rolex crown". OP36 now has easylink, but has no fliplock, and has "laser etched rolex crown".
3. explorer 369 dial is lacquered black, with a small white painted crown logo, and lumes throughout the indicies. op36 dial is more like a sunray black and has lumes througout the indices, except the 12'o clock crown.
4. the explorer bezel is more flat and slanted, while op bezel is more domed.
5. 124270 bracelet is smaller. lug width is 19mm and tapers to either 14-15mm. 126000 bracelet is still the good old 20mm lug width, tapering down to 15.5/16mm. thankfully, both have matte center links. In a way, this makes the explorer look almost like a 35mm watch.
based on what i saw at the AD, I will choose an OP here. Actually I would choose neither, due to the 39mm being the absolute smallest limit for me, but OP here looks bigger than a 36mm, whie explorer dimension is more reminiscent of 34mm rolexes. to me, Op proportion works better. i also believe other dimensions are different as well, at least between the exp1 and op now, and op 36 is more resemblant to the current dj 36, a big 36mm. i even hear that 214270 is thinner than 124270, which annoyss a bit more aslwell.