In the routing model, you choose whether to push work to agents who are Least Active or Most Available. 

Least Active:

If you select Least Active, then Omni-Channel routes incoming work items to the agent with the least amount of open work. 

Most Available:

If you select Most Available, then Omni-Channel routes incoming work items to the agent with the greatest difference between work item capacity and open work items.

But what if there’s a tie between two or more agents? 

In that case, Omni-Channel routes the work to the agent who received a work item the longest ago. Suppose that Agent A received work 10 minutes ago and closed it 2 minutes ago. Agent B received work 8 minutes ago and closed it 5 minutes ago. In this situation, the work would go to Agent A.

Help Articles:

https://help.salesforce.com/articleView?id=sf.service_presence_routing_options.htm&type=5

https://help.salesforce.com/articleView?id=sf.omnichannel_routing.htm&type=5

https://help.salesforce.com/articleView?id=sf.service_presence_routing_options.htm&type=5