Oxygen

Elk Studios places Oxygen on the seabed, running its reels behind a transparent panel over a sunken ship and coral growth, with a calm underwater soundtrack setting the tone before the multiplier system takes over.

The game plays out on a six-reel, three-row grid using a cluster-pays structure rather than fixed lines: five or more matching symbols connected vertically or horizontally anywhere on the grid form a win. Winning symbols then clear away and new ones drop in behind them, a tumble sequence that keeps going for as long as fresh clusters keep forming. Clusters of five symbols pay between 0.1x and 1x the stake, climbing to a 10x-100x band once a cluster reaches fourteen or more symbols. Regular pay symbols and wilds alike can also appear as oversized blocks, in 2x2, 3x3 or 4x4 shapes, occupying multiple grid positions at once toward a cluster.

Eight pay symbols make up the paytable, three lower-value markers and a set of higher-paying sea creatures above them. Three wild types are in play: a standard version that substitutes for any pay symbol, an Exploding Wild that clears a cross-shaped area above and below its own position once it lands in a win, and an X-Blaster Wild that turns a randomly chosen symbol type into a matching cross pattern across the grid. A symbol struck by more than one blast becomes a further wild, and a cluster made entirely of wilds pays at the rate of the top symbol.

Multiplier values build on the grid positions themselves: the first win on a spot adds a bubble worth 1x, and every subsequent win on that same spot doubles it, climbing as high as 2,048x. Those values clear between paid spins but roll over from one free spin to the next once a bonus round is running. Three extra multiplier events can also fire at random on any drop — the Multiplier Strike drops a batch of fresh multiplier values onto the grid, the Collector pulls every visible multiplier value into a single winning spot, and the Dealer multiplies whatever sits inside an adjoining V-shaped block by the number on its own position.

Landing three scatter symbols opens the Bonus Game, ten free spins where any multiplier already built up carries straight into the round, and two or three scatters landing during the spins add three or five more respectively. Reaching free spins already holding a persistent multiplier feature instead unlocks the Super Bonus Game, another ten spins in which both the multiplier values and at least one multiplier feature remain live throughout. A feature-buy menu called X-iter offers five separate purchases: a triple-chance Bonus Hunt spin for 3x the stake, a guaranteed Multiplier Strike for 10x, a guaranteed persistent multiplier feature for 25x, the Bonus Game outright for 100x, or the Super Bonus Game for 500x.

Oxygen pays back 94% at medium-to-high volatility, hits on 36% of spins, and tops out at a 25,000x win. Stakes range from 0.20 to 100 per spin. That hit rate keeps the reels moving at a fairly regular pace between drops, with the largest swings coming once a run of stacked multipliers lines up behind a big cluster.