On center (OC) describes spacing measured from the center of one member to the center of the next — studs, joists, rafters, rebar or fasteners. “Studs at 16 inches on center” means the centerline of each stud is 16” from the next, no matter how thick the stud is. Measuring center-to-center rather than the gap between boards keeps the layout identical whether you frame with 2x4s or 2x6s.
US framing is built around 4 × 8 ft sheet goods, so the standard spacings divide evenly into 48” and 96”: 16” OC (three spaces per 4 ft sheet) for most walls and floors, 24” OC (two spaces) for advanced framing, roof trusses and many 2x6 walls, 12” OC for heavy loads, and 19.2” OC (five spaces per 8 ft) for engineered I-joists. That is why sheathing and drywall edges always land on a framing member.
To lay out 16” OC, hook the tape on the end of the plate and mark at 15-1/4”, then every 16” after that (most tapes highlight 16, 32, 48… in red). Because the first mark starts 3/4” short — half of a 1-1/2” stud — each mark is the stud’s edge and its center lands exactly on 16”, 32”, 48”. Draw an X on the side of the line where the stud goes.