I realized I should just do some research to try to figure out who “O&M” was and it wasn’t long before I pieced together that it is probably the city’s Operations and Maintenance department. This partly explains the brevity of the stamp. Most of these serve partly as advertisements for the contractor, but the city doesn’t need to advertise. Presumably they just mark them for informational purposes instead. I do wish they were dated like most of the sidewalk stamps around town.

My guess is that the city mostly does sidewalk work when they have disrupted the pavement working on some other project in the area.