To find the cost of one ticket to the concert, we first need to determine the total amount paid for the tickets without the insurance.
There are 4 people attending the concert (Oksana and her 3 friends), and each paid $15 for insurance. So the total cost of insurance is:
\[ 4 \times 15 = 60 \]
Now, we subtract the total insurance cost from the total amount paid for the tickets:
\[ 540 - 60 = 480 \]
This $480 represents the total cost for the concert tickets. Since there are 4 people, we now divide the total cost of the tickets by the number of people to find the cost of one ticket:
\[ \frac{480}{4} = 120 \]
Thus, the cost of one ticket to the concert is $120.