To find the cost of one ticket, we need to determine how many people are going to the concert and how much they paid in total for the tickets.
Oksana and her three friends make a total of 4 people.
The total amount they paid for the tickets is $540. Since we need to find the cost of one ticket, we can divide the total cost by the number of people.
\[ \text{Cost per ticket} = \frac{\text{Total cost}}{\text{Number of people}} = \frac{540}{4} = 135 \]
Thus, the cost of one ticket to the concert is $135.