While there will be at least one guaranteed reward when cleared, I know there will be more than one reward for doing other things in the campaign, much like how Octo Expansion had other rewards.
While we most likely do get the Replica of the weapon shown in a blurry image of Side Order, I don't think that weapon is the Octo Shot, considering we already have seen enemy Octolings in Return of the Mammalians already use the Octo Shot in its iconic design. Rather, I think that one of the rewards will be the Octo Shot Replica is the same design we had before, first because that weapon will always be the alternative to the Tentatek Splattershot, just like how the Hero Shot Replica has always been the alternative to the Splattershot. But also because the mode is literally called Side Order, so it makes sense to have at least one campaign weapon Replica not to have its design changed at all.
While we most likely do get the Replica of the weapon shown in a blurry image of Side Order, I don't think that weapon is the Octo Shot, considering we already have seen enemy Octolings in Return of the Mammalians already use the Octo Shot in its iconic design. Rather, I think that one of the rewards will be the Octo Shot Replica is the same design we had before, first because that weapon will always be the alternative to the Tentatek Splattershot, just like how the Hero Shot Replica has always been the alternative to the Splattershot. But also because the mode is literally called Side Order, so it makes sense to have at least one campaign weapon Replica not to have its design changed at all.