On pg 158 of the corebook it states... Anyone that successfully deals Damage to a Toxic creature must make a Toughness Test or suffer Damage not reduced by armour or toughness.

So RAW if i shoot a toxic creature with my longlas from 400m away somehow i am injured?

Yes. Worst rendition of Toxic ever, but yes. Nothing in the "Living Errata" says otherwise.
"Living Errata"...whatever.

I'd recommend changing it to Damage dealt while in melee combat.

Yes.

The Toxic Trait have been crappy in the previous iterations, but boy I think Only War has the worst one yet.

Believe it or not, but in one of the settings (Rogue Trader, I believe), Toxic actually does the same kind of damage as you did to the creature that had toxic. Yup. That means what you think it does. You just hit that beast with a Power Sword, so now you're going to take Energy Damage from it's toxins.

Just.. change it to melee only, or maybe a reasonable splash area, and apply some nice poison. The whole trait is just odd, no matter what book you pull it from.