Overview
Plywood and OSB are both structural wood panels, engineered sheet goods that do the same three jobs on a jobsite: wall sheathing, roof decking, and subfloor. Plywood is built from thin wood veneers cross-laminated with the grain alternating ninety degrees. OSB (oriented strand board) is wood strands compressed with resin under heat and pressure. I have nailed off acres of both, and the thing crews get wrong is treating this like a quality contest. It is not. For most structural uses they carry the same span ratings and satisfy the same building codes, so a panel-rated 7/16 OSB and a comparable plywood are interchangeable as far as the inspector cares. The decision is not strength. It is cost versus how the panel behaves when it gets wet, and on a real build it will get wet. That is the whole argument in one sentence: OSB is cheaper, plywood handles moisture better. Everything else is detail. If you want the dollar breakdown, that lives in the plywood and OSB prices page; if you want how the panels go into a floor or wall assembly, that is the subfloor and sheathing guide. This page is the head-to-head material call.
What They Are
Plywood is an odd number of thin wood veneers (plies) stacked with the grain rotated ninety degrees layer to layer and glued under pressure. The cross-grain construction is why it resists splitting and holds a screw well in any direction. A typical sheathing or subfloor panel runs three to seven plies depending on thickness. OSB is different by design: large wood strands, roughly the size of your hand, are sprayed with a waterproof resin and wax, oriented in layers (face strands run long, core strands run cross), then pressed under heat into a dense mat. Because it uses small, fast-growing trees and the whole log instead of premium peeler logs, OSB is cheaper to make and more uniform, with no internal voids, knots, or soft spots. Both come in the grades that matter for framing: sheathing grade for walls and roofs, and tongue-and-groove subfloor grade (often called Sturd-I-Floor) for floors. OSB also comes in larger sheet sizes, which is handy on long walls. Strength-wise the strand orientation in OSB is what lets it match plywood's span numbers. They are built differently, and they get rated to the same performance standard.
The Moisture Difference (The Big One)
Here is the difference that actually shows up on a jobsite. Plywood absorbs water relatively fast, but it also dries fast, and when it dries it returns close to its original dimensions and thickness. Get plywood wet, let it dry out, and it is mostly fine. OSB does the opposite. It resists initial wetting because of the resin and wax, so a quick rain shrugs off. But once water gets past that surface and the panel saturates, it absorbs slowly, swells, and the swelling concentrates at the cut edges where the strands are exposed. The worst part: OSB does not fully recover. Those swollen edges stay proud even after the panel dries, leaving a ridge you can feel through flooring and see along roof seams. The failure mode is prolonged wetting, the kind that happens when a roof sits under tarps for weeks, when a subfloor takes rain during a stalled build, or when flashing leaks for a season. A single storm rarely kills either panel. Weeks of repeated wetting and drying is what wrecks OSB edges. That is why the panels that worry me most are the ones exposed to weather during a long build, where drying speed and edge stability decide whether you sand ridges later or sleep fine.
Strength, Stiffness, and Cost
Both panels meet the same structural span ratings, stamped APA rated, so for code purposes they are interchangeable across most sheathing, decking, and subfloor uses. The differences are at the margins, but they are real. Plywood is a bit stiffer for a given thickness, holds fasteners slightly better, and weighs less, which your back notices by the end of a day of decking a roof. OSB counters with uniformity: no voids, no knots, no soft delaminated corners, so panel to panel it behaves predictably. It comes in larger sheet sizes, and most important, it is consistently cheaper. That cost gap is not small at production volume, and it is the entire reason OSB dominates new construction. A tract builder framing dozens of houses sheathes everything in OSB and saves real money per house, with the structural performance the code requires. When I see plywood on a production job it is usually in the spots where someone made a deliberate moisture call, not across the whole house. Strength is a tie for code. Cost is the lever, and it points at OSB.
Where Each Wins
OSB wins anywhere it stays dry and cost matters, which is most of a typical house. Wall sheathing behind a properly installed weather-resistive barrier and siding, gable ends, most general sheathing, and the bulk of production framing all go to OSB without a second thought. The panel is protected, the price is right, the code is satisfied. Plywood earns its premium in the wet and exposed spots. Roof decking in a wet climate, where the deck may sit through rain before dry-in and where any future leak gets a panel that dries and recovers. Subfloor in bathrooms, laundry rooms, and anywhere under tile or a wet room, where a slow plumbing leak or a mopped floor should not leave you with a permanent edge ridge. And anything that will sit exposed during a long or stalled build, where weeks of weather are likely before the assembly is closed in. The deciding factors are simple: long exposure, repeated wetting, fastener demand. When those stack up, plywood's faster drying and edge stability are worth paying for. When they do not, OSB is the rational default.
How to Choose
My rule is short. Default to OSB for cost on any dry, protected assembly: most walls, most general sheathing, anything that gets covered quickly and stays dry. Pay up for plywood where moisture exposure, drying, or fastener hold actually matter: roof decking in a wet climate, subfloor under tile or in wet rooms, and any panel that will sit exposed during a long build. That is the whole decision tree. But here is the part crews skip: the assembly matters more than the panel. A roof dried-in on schedule with good flashing and underlayment, a wall behind a properly lapped weather-resistive barrier, a floor that does not take standing water, all of those protect either panel for the life of the building. A sloppy install soaks OSB and plywood alike, and a great panel inside a leaky assembly still rots. Both panels also need proper gapping at the edges and ends so they can expand without buckling; skip the gap and you create the exact ridging you were trying to avoid. Choose the panel for the moisture risk, then build the assembly that keeps water out. Cost detail is in the plywood and OSB prices page; assembly detail is in the subfloor and sheathing guide.
FAQ
Is OSB as strong as plywood? For code purposes, yes. Both carry the same APA span ratings for sheathing, roof decking, and subfloor, so they are interchangeable structurally in most uses. Plywood is marginally stiffer and holds fasteners a touch better, but OSB meets the same engineered performance standard.
Why do builders use OSB if plywood handles water better? Cost and uniformity. OSB is consistently cheaper, comes in larger sheets, and has no voids or knots, so at production volume it saves real money per house with the structural performance the code requires. Most of a house stays dry once it is closed in, so the moisture advantage of plywood never gets tested there.
What happens when OSB gets wet? It resists a quick wetting, but prolonged saturation makes it swell, especially at the cut edges, and it does not fully return to its original thickness after drying. Plywood absorbs faster but dries faster and recovers close to its original dimensions. That non-recovering swollen edge is OSB's main weakness.
Should I use plywood or OSB for a roof? In a dry climate with a fast dry-in, OSB is fine and most roofs use it. In a wet climate, or if the deck will sit exposed before underlayment goes on, I lean plywood for the faster drying and edge stability. Either way, get the roof dried-in promptly and flash it well, because the assembly protects the panel more than the panel protects itself.