It's not centralized and I think only some exchanges have a COB. It's up to your broker how the order is sent. Mine provides the option to specify which exchange to send the order to.
Each COB, like each Option exchange, is independent. If your broker/software offers you the choice, you can direct your option orders to the option exchange of your choice. If you have a SMART route, the route chooses. For our customers that want to choose their option exchange, we offer Sterling Trader Pro, Realtick and for PMA only CBOE Silexx and WEX.
Thanks for this. I could be totally wrong here, but I feel like IB kinda does this now - at least I think they say they do. My vision for how it works is that - when I enter a three/four legged order - that IB may market it as complex but then also works the individual legs separately. If the system catches a great deal on one leg and can grab the others and still achieve my limit price for the combo, then it executes all the legs at once. I may have dreamed all of this.
I think IB will do that, but I suspect that they will take less execution risk than I would be willing to take -- they won't leg into part of the combo without knowing they can immediately fill the other half and still meet the required limit. For example, if I'm trying to sell a 250/255 call back-ratio spread and I'm able to buy the 250 / 255 bull call vertical part of the spread now at an attractive price, I'd be willing to take on that part of the trade first and then sell the extra 255 call later with the expectation that the underlying will rally a bit sometime over the rest of the day. Alternatively, maybe someone really wants to get filled on a short 210/215/250/255 iron condor. So I take the order, short the 255 call and then look to cover the 210/215 short put spread later. That's the kind of trade I might be willing to do depending on my outlook for the underlying during the expiration period.
I experimented with that for a while some years ago, but it didn't work out for me. I would either get a pretty good fill waiting for the other leg - sometimes even a free spread or a credit with an immediate locked in profit; or it would go horribly against me and I'd wind up doing the other leg(s) at a big loss while waiting. I hope it works out better for you.