I was particularly concerned that with the 18mm lugs that an 18mm strap whilst it would fill the gap, may leave a bit of a visual gap whereas something a little wider say 19mm would provide a more snug fit as the leather would have plenty of leeway for squishing 0.5mm either side. When I first got this watch back in the 80's having been handed down to me after my Dads passing it had a metal band which from memory was a crappy speidel which I removed and it really hasn't had a band on it since, mostly because I wanted a genuine Omega metal strap but finding one proved too hard to get back then. I guess 35 plus years is too long to have it sitting in the drawer so I finally decided it needs a strap and leather does look good on these, one of these days I may find the appropriate Omega metal band but I don't want to wait another 35 years so a leather one it will be.
They are designed to take an 18mm strap so there shouldn’t be any gaps. Here’s mine on an 18mm (16 buckle) Hirsch Duke. Also more available choice at 18mm
That looks a good fit, so 18mm it will be. Like that brown strap, was thinking either black or brown, either one will look good I am sure on the silver Seamaster.
If you want to also look for a metal bracelet after you get the leather strap, keep an eye out in the forum “watch parts and accessories for sale” section. The “beads of rice” style braclet goes well with your Seamaster DeVille watch and often show up there. For this watch style, I like the 7 interior link a bit better than the 5. As an example of the style, here’s one that recently sold: https://omegaforums.net/threads/ss-omega-beads-of-rice-bor-18mm-no-15-end-links.90909/
My (rather anal) rule of thumb is that if I’m wearing a suit and doing something formal I prefer a black strap, and if it’s a jeans day, a brown. Of course, sometimes I wear black with jeans and brown with a suit
Normally I would like to see black with silver and yes they are more formal but they have become so ubiquitous that I am leaning toward a brown strap, that and I almost never get very dressed up these days tending to be in jeans 90% of the time.