Looking to use an Omni cartridge bottom bracket in a vintage Italian frame. The rear spacing is 126 and i will be using a Campagnolo Super Record double crankset. I just want to confirm that a 115mm spindle length is correct? This was the case on every other similar frame but i was wondering if someone can confirm this.

probably (But not 100% certainty): the post-'79 super record double crank in a 70mm shell (Italian) would be 115
a pre-'79 in the same shell would take 113
I assume your "vintage Italian frame" with 126 spacing will be using the post-'79 crank and I assume this Omni BB has ISO taper.
You know what they say about assumption...

Also, the Campy cranks take an asymmetric spindle where the drive side is 2mm longer than the NDS.

Im thinking it should work then it even says asymmetrical axel.
https://www.ebay.com/itm/OMNI-Racer-T...item564df58074

Should work, though I personally wouldn't spring for the ceramic bearings. Overhyped and not worth the dosh. I would either go with the steel version, or this one:

https://www.ebay.com/itm/OMNI-Racer-S...item33839e4739

Hmm, what makes those better than the $25 Shimano units? (Apart from the CF and anodized bling that you won't see when it's installed, I mean.)

They are ISO, and they don't say ShimaNO on them?

D'oh, for some reason I thought the 70mm Italian-threaded ones were ISO.

Perhaps they are, never looked.

They're not. I guess Shimano expects you to ditch your Campy crank when you put their BB in an Italian frame...

Besides saying shimano they also only weigh 170 grams vs the 285 of shimano's $25 offering.

Which is a 1/4 of a pound difference, in less-impressive units. (Don't mind me, I just like poking fun at weight weenies. )

115 should be about right the original BB and spindle if you have it measure it is I believe 113or 114 with a slight drive side offset so 115 none offset should work fine. This is assuming your using basically vintage 126 spaced drivetrain with a 6/7 speed rear and friction shifting.

Bingo. Now is the question of how much of an actual difference will the ceramic bearing make vs steel?

a difference of approximately $100

None ceramic is speced to the same as steel there shouldn't be any diffierences and those you find should should be will under .1 mm for complete set well within torence provieded you have the right size.

+10 Loose ball classic properly done will match or beat most ceramic for practical purposes $10 vs $100.

A 110mm Shimano (or other JIS) symmetrical bb will have an effective spindle length of 115mm or so in ISO/Campag terms, since the taper is thicker.

What is called a "2mm offset" (where the drive side end of the spindle protrudes 2mm further than the non-drive end) is really just a 1mm shift to the right of the entire spindle.
The right end gains 1mm, while the left end loses 1mm, so the right end is thus 2mm longer than the left.
But the increase in driveside chainline here is only 1mm versus the symmetrical bb (assuming same-length and same-taper spindle).

So considering all this, the 110mm symmetrical Shimano cratridge bb should give about the same chainline as the original Campag bb.
And since the Cartridge bb needs no lockring, any amount of spacers can be used under the driveside (fixed) cup flange, thus mimicking the offset of the Campag bb while giving any amount of needed chainring clearance at the chainstay.

Many Chinese-made bb's use a smaller, flexier 16mm spindle diameter, in order to allow bearings with thicker section to fit into the cups.
Shimano's cartridge bb's integrate the inner and outer bearing races into the spindle and body, respectively, allowing larger bearings together with a very strong spindle. This might explain some of their heavier weight than some other's advertised weight.

In all of my years seeing ceramic-ball bearing cartridges in use, I've never noticed any improvement in performance or longevity, particularly when compared to Shimano's parts, although the balls themselves are lighter than steel.

Actually the difference between the ceramic and steel is $50 not that much considering its built for racing. Plus an equivalent weight (campy) Ti bracket is $200+ for under 200grams.

I was told ceramics offered lower rolling resistance. Is this true?

Replaced the old BB in my son's 86 Team Fuji, Sugino BB, Cups, Spindle, the works, with a nice, fresh out of the box Suntour Superbe unit...weighed just a hair less, but the quality of the finish was magnificent! While doing that, I compared the weight to that of a Velo Orange cartridge (that was going in my MODERN Felt-85), the cartridge was an once and a half lighter that that BEAUTIFUL Suntour! I was surprised, I was expecting the opposite!

No surprise that in all these years that a newer bb benefits from improved design efficiency and thus lower weight.

But is the FELT's bb spindle a full 17mm diameter? I suspect not.
And is the FELT's bb as easily serviceable as the Suntour's design(?) (won't know until you try).

But in truth, the Suntour cartridge bb is an antiquated design with it's circlip grooves cut into the spindle.

Yes, the new BB Cartridge seems to be a hair smaller diameter. There is NO SERVICING of the cartridge, you just use it and throw it away! Indeed, an easier instal, makes it quicker and less fuss for an LBS...less thinking involved. The Felt, which I got used, BB started going click...pop....and had lateral play....NO idea of how many miles, but it was about 12 yrs old. The VINTAGE BB, was starting to softly grind...it was 27 yrs old. I had serviced it when we picked it up last year, and noted that the spindle and cups were both pitted...and NO ONE in town had anything but cartridges to replace it. Got lucky and found the Suntour at a Garage Sale, in it's BOX, for $12...Superbe Pro BABY!!!! Nice....

I mis-read your earlier post, thought you bought a Superbe cartridge bottom bracket.

The conventional Superbe Pro bb with the rubber seals is a Sugino-made unit that should last a long time.
Those were 109mm or something?

The Suntour Superbe Pro (Conventional) replaced an old sugino in my son's bike. At about the same time as I was hunting for a cartridge replacement on my felt...NO NAME cartridge got replaced by the VeloOrange Cartridge.