The Oz score takes into account 2 factors:  

1.) Amount of competitive content 

2.) Overall search volume for semantically-related keyword terms. 

For example: 

Topic A may have 1000 unique searches per month, but 99 pieces of content already out there on the topic. 

Therefore, if you create 1 more piece of content, your share of monthly unique users are 10 (1000 uniques/ 100 pieces of content.) 

Topic  B may have only 180 unique searches per month, but only 2 pieces of competitive content already out there. 

For topic B, your share of monthly unique users are 60 (180 uniques/ 3 pieces of content).  

Therefore, given other factors are constant, Topic B will outperform Topic A by 600%.