Manchester City icon Kompany left the Premier League giants at the end of last season to return to his first club in a dual capacity following Anderlecht's disappointing fourth-place finish in 2018-19.
But their 2019-20 opener is proof Kompany has much work to do, as Oostende – who ended last term 14th in the 16-team division – came from behind to take the spoils.
The home fans unfurled an impressive tifo in tribute to Kompany before the match, with the banner reading "left as a prince, returned as a king".
And it all looked to be going so well when Michel Vlap, a new €8million signing from Heerenveen, gave Anderlecht the early lead with a fine finish inside the area in the 13th minute.
But Oostende breached the hosts' defence – which included Kompany – soon after, with former Anderlecht midfielder Ronald Vargas converting Robbie D'Haese's low cross.
Samir Nasri was introduced as a substitute in the second half, with Kompany's former City colleague making his debut after joining on a free transfer.
But Oostende had designs on sealing all three points, and Fashion Sakala netted against the run of play 15 minutes from time to inflict a first competitive defeat on Kompany as player-manager in his debut outing.