To find the cost of one ticket to the concert, we first need to determine how much money was spent on the tickets alone, excluding the insurance.
1. **Calculate the total insurance cost:**
Since there are four people (Oksana and her three friends), and each person paid $15 for insurance, the total insurance cost is:
\[
\text{Total Insurance} = 4 \times 15 = 60 \text{ dollars}
\]
2. **Determine the total amount spent on tickets:**
Now, we can find out how much was spent on tickets by subtracting the total insurance cost from the total amount paid:
\[
\text{Total for Tickets} = \text{Total Amount Paid} - \text{Total Insurance} = 540 - 60 = 480 \text{ dollars}
\]
3. **Calculate the cost of one ticket:**
Since there are four people, we divide the total amount spent on tickets by the number of people:
\[
\text{Cost per Ticket} = \frac{\text{Total for Tickets}}{4} = \frac{480}{4} = 120 \text{ dollars}
\]
Thus, the cost of one ticket to the concert was **$120**.