I would guess late 1950s to early 1960s. Hard to say since there's lack of information. First, I would scrutinize that serial number carefully to see if/how it fits within the Olmo Serial Number Registry I compile [ Olmo Serial Number Registry, Peter Brueggeman ]. To me, it looks like C966 or O966 but it is hard to see in those photos. I would work on it with a toothpick to see if I could dig some surface dirt out and delineate the letter and numbers better. I would want to know which parts on it were really old for dating input. The brakes are very good quality, being the anodized Weinmann 999 Vainqueur Deluxe brakes. Can't tell if they are single spring or double spring though, with the cutover in Weinmann catalogs being 1962 as mentioned. Even if they aren't original to the bike, they date the frame back to pre 1962/1963. The hubs look old and whatever they are and the rims could be a clue to dating. I can't tell for certain but there seems to be no braze-ons on the top tube for the brake cable? From the Brianza lugged Olmos I've seen, they seem to all have top tube brake cable braze-ons. Perhaps really old ones do not have these braze-ons. If you want to converse on this further, you can email me at the email address at top of the Olmo Serial Number Registry..... Peter