Chernobylite

This is missable depending on the choice you make on the 'Traitor' quest.

It takes not one but two missions to recruit Olga. The first one will be during the mission "Traitor," Olivier will ask you to kill the Traitor. If you want to recruit Olga, you should let Kostanty aka the Traitor go free instead.

The second mission will be when Olga contacts you through the radio giving you the quest "A voice in the woods,".Go to the quest and talk to Olga, she will have you go retrieve some food supplies. Go to the marker and deal with the NAR soldiers there. Go downstairs and release Kostanty and a little further there you can retrieve the food supplies. Go back to Olga's camp and agree to help Kostanty, talk to Olga and she will be so happy that she will be available to recruit.

Olga's Training :
Discipline: +3 Health
Organization +6 Inventory Slots
Perfect Organization: +12 Inventory Slots
Control: +25% Assault Rifle Damage
Perfect Control: +50% Assault Rifle Damage

If you do miss this the 1st time around just visit your Radiation Chamber. Using this automatically kills Igor (self-annihilation). You’ll appear in the Memory Timeline area, but you won’t lose any loot and you will be able to change your choices for 3 chernobylite each time.