To find the cost of one ticket to the concert, we first need to calculate the total cost of the tickets without the insurance.
Since there are four friends (Oksana and her three friends), the total insurance cost is: \[ 4 \text{ people} \times $15 \text{ insurance per person} = $60 \text{ total insurance cost} \]
Now, we subtract the total insurance cost from the total amount paid for the tickets: \[ $540 \text{ total cost} - $60 \text{ total insurance cost} = $480 \text{ total cost of tickets} \]
Next, we divide the total cost of the tickets by the number of people to find the cost of one ticket: \[ \frac{$480 \text{ total cost of tickets}}{4 \text{ people}} = $120 \text{ cost per ticket} \]
Thus, the cost of one ticket to the concert was $120.