Hey everyone,
I'm about 3/4 of the way through my hardwood installation of about 1000 sq ft. I'm using 3/4" thick x 5" wide oak which I am fastening with 2" pneumatic flooring nails into a CDX subfloor.

So far I've been driving my nails around 12" apart throughout the installation. I just read the recommended distance apart for the nails is 6" which made me panic a little! What is the likelihood this will cause a problem?

Since I'm so far along, the only thing I could think to reinforce would be to face nail everything with my 15 gauge or 18 gauge nailer. The 18 gauge brads would be barely noticeable I think, but not sure if they'd give enough additional strength. The 15 gauge nail heads would show and I'd have to putty holes all over the floor -- not a great look but more strength.

My hope is that I can get away with how I've done it so far, if it's not a huge risk. If not, I appreciate any advice, thank you!

I usually fasten two and a quarter inch 8-12 inches apart. For three and a quarter it's 8 inches. With a wider board you are using one half of the nails of a narrow strip. If your flooring is acclimated correctly it will not swell or shrink very much over the year. It's most likely going to be fine. The nails have some coating from how they were collated which acts like glue when they are driven. If you do get any squeaks from a loose fastener you can always add a top-nail and putty it. On wider planks I like to staple or cleat into the tongue at the ends of the plank to fasten more securely.
If you wait a few weeks before sanding you may find an area where you could add a nail.
If this flooring is pre-finished then wait to see if any squeak would be bothersome, if you do get a squeak. Get a couple of colors of putty if you go by the floor store.

Thanks so much Pete! That sounds like a great approach. I'm fine with combating squeaks that way, I think my bigger concern was that the boards might be more likely to pop up and bow in the summer months with fewer nails. Is that so, or probably not a concern?

My wood is acclimated to the rooms I installed, but its winter so on the drier side. I planned to run a dehumidifier all summer to try and keep it a relatively stable humidity, but I don't have central A/C.

Summer months can be hot and humid so having a dehumidifier handy is a great idea. Slight cupping may be expected. Buckling will happen if there is a water spill.