To find the cost of one ticket to the concert, we first need to determine how much of the total payment was used to pay for the tickets themselves.
Oksana and her three friends total to 4 people, and they each paid $15 for insurance. So, the total cost of insurance is:
\[ 4 \text{ people} \times 15 \text{ dollars/person} = 60 \text{ dollars} \]
Now, we subtract the total cost of insurance from the total amount paid for tickets:
\[ 540 \text{ dollars} - 60 \text{ dollars} = 480 \text{ dollars} \]
This $480 is the total cost for the concert tickets. Since there are 4 people, we divide the total cost by the number of people to find the cost of one ticket:
\[ \frac{480 \text{ dollars}}{4 \text{ people}} = 120 \text{ dollars} \]
Therefore, the cost of one ticket to the concert was $120.