Belgrade carries more OktagonBet counters than anywhere else in Serbia, and they are spread rather than clustered: a few sit in the older parts of the centre, but most are out where people actually live — Zvezdara, Vozdovac, Palilula, and the newer blocks across the Danube.
That spread is deliberate. A counter earns its place by being the one you pass on the way home, not the one you make a trip to, so the network follows tram lines and neighbourhood high streets rather than the tourist map. If two addresses below look close together, they usually sit either side of a busy road that would take twenty minutes to cross on foot.
Each address is a place you can walk into, put money on a ticket, and take a printed slip away with you. Opening hours run late at most of them — the majority to 23:00, several past midnight — but they are not identical, so it is worth checking the one you are heading for. Every venue is 18+ and staff will ask for identification whenever there is any doubt.