OktagonBet runs two kinds of room, and they are not interchangeable. Knowing which one is on the corner saves a wasted trip.
The betting shop
A kladionica is built around the board: football, basketball, tennis and the rest of the week's fixtures, with odds updated as the market moves. You pay in cash, take a printed ticket, and that ticket is the record — keep it, because it is what you present to collect. Most shops carry screens for live events, and the counter is staffed by someone who reads those markets every day.
The slot club
A slot club is a room of machines rather than a counter with a board. Three locations in the network are of this kind, in Lazarevac, Mladenovac and Šabac. The rhythm is different — sessions rather than fixtures, and no ticket to keep — and so is the crowd. If you have only ever used the betting counter, it is a genuinely different visit.
The rules at the door
Both are 18+, without exception, and staff will ask for identification when there is any doubt. Gambling in Serbia is licensed and supervised by the Ministry of Finance through the Uprava za igre na sreću; a licensed venue displays that fact, and it is reasonable to expect it.
One more thing worth saying plainly: a betting shop is entertainment with a price, not a way to fix a shortfall. Decide what an evening is worth to you before you go in, and treat that figure as the limit rather than the starting point. If it stops feeling like a choice, stop — the counter is not going anywhere.