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Early Morn Transport to Zaventem
I don't think so. The 830 bus from Overijse centre to Zaventem doesn't start until about 5 am.
Uber?
The coverage map on their website covers some of Overijse. Might be worth a try.
maybe the last train (SNCB) or STIB/De Lijn the night beore? If you can get in at around 1am, then the wait would only be three hours...take a flask of coffee and a good book or make sure you have some entertaining games on your laptop...