From my perspective, the main reason for choosing the OB7s over the NX-Oticas would be price. The Oticas are considerably more expensive. I haven't heard the OB7s so I cannot make any comparisons between them and the NX-Oticas. When it comes to comparing/contrasting the OB7s and NX-Oticas, Jay (Captainhemo), will be a good source of information since he has had both. Maybe he'll chime in on that score.
With subs you will get the best blend (especially with the NX-Oticas) with an OB servo sub for each channel. This will produce a main speaker combo that plays full range (20Hz-20kHz). You will still need one or more sealed/ported subs for movies. OB subs do not handle special effects well. The infrasonics cause them to bottom out.
The O-3's were designed as surround speakers for the OB-5/7 and would work just as well with the NX-Oticas.
On the non-OB side you mentioned the N3 transmission line towers. These are also exceptional and would be a big step up from your current setup. The O-3's will also work for rear surrounds for N3's.
I can give you my opinion on the N3 vs the MTM monitor version of the NX-Otica. I have both in different rooms. The N3's are mated with a GR/Rythmik 12" sealed servo sub (F12G if bought from Rythmik). The NX-Otica monitors are mated to dual 12" OB servo subs for each channel.
The N3's are also in a 4.1 surround system (no center channel speaker). While the N3's are designed to be -3dB at 40Hz, my in room response sounds more like mid 30's (one of these days I'll get around to measuring it). With this I can listen to music recorded in 2 channel in stereo mode and multi-channel for multi-channel sources.
The N3's are a real pleasure to listen to; smooth, detailed, full, jut an all around joy. The only speakers I have found I like better are my OB's (either the LGK Wedgies or the NX-Otica MTM's). OK, that's not exactly true. The Neo 3/10 line array with the triple 12" servo towers for each channel Danny built is the best I've ever heard but for a mere mortal the Wedgies and Oticas are my reference point. As good as the N3s are, the OBs are just more. But then the OB setup (including subs) cost twice what the N3s did.
Up grade path would depend on whether you decided to go with the transmission line or OB for your mains.
If you go with the transmission line then start with the N3's and move your A/V3's to surround. You already have an N3S so you have the matching center. Next consider a second sub if you need it in your room to even out base response (one sub works fine for me since I don't have multiple rows of seating, just me and my wife). Eventually build a pair of O-3's for surrounds. When you add the Atmos speakers N2X's would be the matching speakers. Danny has also suggested LGK 1.0's as ceiling mounted speakers in a different thread.
If you decide to go OB, either OB-7 or NX-Otica, again start with the mains and move the A/V3's to surround duty. While you can keep using the N3S as center i would recommend experimenting with it both in and out of the system. I find the NX-Oticas (and the smaller Wedgies) to image so well and clearly, dialogue sounds better without a center channel speaker.
Next add OB subs to the main channels. Both at once if your budget allows or one at a time if it doesn't. If you start off with one, set it in the middle and combine both channels.
Then add O-3's for surrounds. Again, N2X's or LGK 1.0's for ceiling speakers.
I'm sure there are other options and paths as well.