One would think there should be the imperative form of occhiare in the place of occhio, which is a noun.
I might be completely off here, but I imagined it would be a shorter version of “[keep an] eye on [x]”, so indeed the noun, instead of a verb, like for instance “[keep your] eyes on the prize” or such.
The specific “occhio alle dita” expression isn’t listed under “occhio” on Treccani, but there are many similar expressions/constructions listed there, whereas “occhiare” is just listed as an uncommon variation of “adocchiare”.
Plus there are many variations of “[keep an] eye on [x]” on Tatoeba, where I could imagine in the imperative form “dare/tenere un occhio a” could easily be abbreviated to “occhio a”. But that was just my interpretation of things, and like I said, I could be way off with that.
Ciao @sindaco “Occhio a” rings a big bell, as does “tenere occhio”.
Grazie per questo…