Thanks for the replies concerning OTA. I just have the feeling that this concept, regardless of where it originated, has mainly been embraced by the Olicity fandom. I'm fully aware that this is a very Felicity/Olicity/-oriented forum, but I've seen lots of posts (both on FB and other forums) where posters don't even know what OTA stands for, and therefore have to ask what it means. These posters are baffled when they learn that some fans consider the O/F/D constellation as more "core" and "special" than the many other constellations of the team that we've seen over the course of five seasons, and that these characters therefore deserve to be singled out and regarded as a kind of "Holy Trinity" (a term I've seen used by OTA fans!). I understand if David feels that Diggle's relationship with Oliver and Felicity is more special than his relationship with Roy, Laurel, Sara and Thea, but it's still odd to see an actor adopt tropes and notions which are very much connected with a specific fanbase (regardless if they were originally coined by some of the actors). I'm saying this because hardly anybody outside the Olicity fandom uses the OTA moniker, or gives it as much weight and importande to THIS particular version of the team as the Olicity shippers.
On the other hand, it's fully plausible that David Ramsey is expressing the party line. I mean, 2/3 of the characters that the "comic book Fanbois" would regard as the canonical Team Arrow members have left the show (Roy, Laurel). This means that the core team in this AU version of the comic book Green Arrow consists of Oliver/GA and two made-for-the show characters, namely Diggle and Felicity. In that sense David Ramsey is just expressing the current direction of the writing....it's Felicity and Diggle who are Oliver's REAL team in the CW "Green Batman" version of the GA comics (because OQ himself shares more characteristics with Nolan's Batman than with the chilli-making, humorous, Leftie GA of the comic books).
I'm fully aware that my POV is highly unorthodox and maybe even scandalous/reprehensible on this forum, given its pronounced Felicity/Olicity/OTA slant. However, my hope is that there is enough acceptance for diverging opinions that I won't be attacked for saying something that is quite clear to most non-shipper fans, namely that OTA's prominence and greatness is not obvious to all viewers, and that the show's own emphasis on this aspect is not entirely a blessing.......at least if you believe that an adaptation should retain at least some of the elements from the original sources. Of course, if you regard "Arrow" as an independent story, rather than as an adaption of an original source, you could say that the core of this independent story are Oliver and Felicity and their love story and the Oliver/Diggle/Felicity crusade against crime and Big Bads. In this case the elements borrowed from the DC comic book lore is just the icing on the cake, so to speak. To my mind that would be the most accurate description of the current incarnation of "Arrow".