Graziano Pellè and Jay Rodriguez each scored a hat-trick as Saints cruised past fourth-tier Dutch opposition in the form of KVV Quick.
A crowd of roughly 3,500 turned up to witness the Premier League side in action in western Holland, with Pellè and Rodriguez both starting in Koeman's team, as did two summer signings - Cédric Soares and Jordy Clasie.
Steven Davis got Saints off to a flying start when he tapped in from close range in the opening 60 seconds, after an inviting header from James Ward-Prowse.
Saints never looked back, banking their second soon after when Pellè got on the scoresheet just after the ten-minute mark. The former Feyenoord frontman muscled off defensive competition for a bouncing ball that had looped into the air after a miscued clearance. He then took two touches, preventing the ball from touching the pitch before slamming a low effort into the KVV net for 2-0.
Clasie showcased his playmaking abilities soon after, lifting a through ball into the path of Rodriguez, who finished calmly to put Saints three goals to the good.
Three soon became four, with Rodriguez netting again. This time his chance was rather more straightforward, after a great save from Gijs Kukkerts saw the ball roll into the path of the England forward, leaving him with an easy finish.
Pellé then matched Rodriguez, as he scored his second of the evening. This time a fizzing low cross from Cédric proved too difficult for the KVV Quick defence to deal with, with Pellè side-footing the ball past Kukkerts.
Davis then followed suit, scoring his second goal in the 39th minute to make the scoreline read as six-nil. This time the Northern Irishman benefited from a smart lay-off from Jack Stephens - who spent last season on loan at Swindon Town - and fired past the KVV stopper.
Pellè grabbed his hat-trick before the half had concluded, seizing onto an inviting pass before rounding the goalkeeper and coolly slotting into an empty net.
Saints went in seven goals up at the interval, with Ronald Koeman making five changes.
Josh Debayo, Jordan Turnbull, Sam McQueen, Lloyd Isgrove and Juanmi emerged for the second half, which saw the end of the evening for Cédric, Matt Targett, Clasie, Davis and Pellè.
The 51st minute saw the eighth goal of a free-scoring encounter, when a short corner from Ward-Prowse and Juanmi led to Turnbull powering a header into the back of the net.
Juanmi then saw an effort saved, but he would soon have a goal of his own. Rodriguez, in line for a hat-trick, unselfishly squared the ball to Juanmi to enable the Spaniard to score Southampton's ninth goal, in the 61st minute.
Dušan Tadić then came on for Ward-Prowse with 19 minutes of play remaining.
Saints were forced to wait until the 86th minute for the arrival of the goal number 10, but it arrived in fantastic fashion when Rodriguez set himself up to smash a swerving volley inside the hosts' near-post.
Next up for Southampton is a trip to Rotterdam on Thursday evening, where they will face the former side of Koeman, Clasie and Pellè, Feyenoord, in front of a sell-out crowd of 51,000 at De Kuip.
Southampton: K. Davis; Cédric (Turnbull - 45'), Stephens, McCarthy, Targett (Debayo - 45'); Reed, Clasie (McQueen - 45'), S. Davis (Isgrove - 45'); Ward-Prowse (Tadić - 71'), Pellè (Juanmi - 45'), Rodriguez.
Unused Substitutes: Gazzaniga, Martina, Long.