For a flight, the straight line geographic midpoint coordinates are 50° 52' 59" N and 2° 58' 55" E.
The city at the geographic halfway point from LIL to OST is Zonnebeke, Belgium.
Travelmath helps you figure out the midpoint between two locations based on the driving directions from each starting point. You can find the closest town that is an equal distance from two cities. Use this tool to determine the best city to meet, or to look for interesting stops along the way if you're planning a long road trip and you need to take a break or stay overnight. Search for hotels at the midpoint city to split up your drive, or explore other nearby cities and discover local towns on your trip. If you're meeting a friend halfway in-between, you can figure out how far each person has to drive and how long it will take to arrive at the center. Even if you're separated by water, you can still calculate the straight line geographic midpoint to determine the closest flight distance.