OVR consists of 2 components: 

[essential] Stars - represent the player’s base quality. It is variable within one season and from one season to another.
[optional] Tiers - represent a permanent key attribute increase that lasts until the player’s retirement. 

Stars
Every player has a base quality and a number of stars in their profile. You can increase your players' stars during the training sessions and live matches, in the Player Academy, or by obtaining various live event rewards and more. Additionally, in case of a seasonal League promotion, the number of stars will be reduced by 1, and the OVR will decrease by 20. 

Tiers 

Player Tiers give a permanent increase on key attributes of a player. The Tier increase on the OVR depends on the number of the player's key attributes. The more white attributes, the greater the OVR increase.
All players are eligible for a Tier upgrade, regardless of their number of stars/base quality. Even though every player is suitable to have the first Tier applied, please note that Tiers must be activated progressively (e.g., a player must have a Rare Tier applied to be eligible for the Elite Tier).