OVR, or overall rating, is the single number a basketball game uses to sum up how good a player is, combining their individual skill ratings into one figure.
In HoopSim a player's overall is a weighted summary of their individual ratings, weighted for the position they play, on a scale that tops out at 99, which is why a center and a point guard with the same skills can carry different overalls. The number you see is never the true one: every grade in HoopSim is your scout's read, shown as letters or numbers depending on the league's display setting, and a better scout gets closer. You know your own players best, and a read sharpens when a player joins you. Playing out of position costs a player effectiveness in the sim, which is a separate thing from how the overall is computed.
Rulebook: You never see true ratings. Related: scouting fog, potential.