Oblate field-reversed configuration (FRC) plasmas are sustained for up to 350 micros, or approximately 15 poloidal flux-confinement times, in the magnetic reconnection experiment. The diamagnetic equilibrium is maintained in argon plasmas as a balance of an inward pinch and outward diffusion. Numerical and analytic models show that the observed stability is provided by a combination of plasma shaping, magnetic diffusion, and finite-Larmor radius effects. FRCs formed with lighter ions, which benefit less from these stabilizing effects, succumb to rapid instability and cannot be sustained.