Oberin
"Oberin is an online fantasy role-playing game and virtual community, available exclusively for Macintosh. Oberin takes place in a persistent, ever evolving world -- your character may come and go, but the world continuously runs."
Free to play indefinitely.
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OGame
"You start out with just one undeveloped world and turn that into a mighty empire able to defend your hard earned colonies. Create an economic and military infrastructure to support your quest for the next greatest technological achievements. Wage war against other empires as you struggle with other players to gain the materials."
Free, with option to pay for additional features.
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Ogre Island
"Do you have what it takes to stand against fierce monsters? To explore winding, trap filled dungeons? To journey from the mountains to the sea and through the vast wastelands?"
Free, with option to pay for additional features.
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Online Boxing
"Welcome to Online Boxing, the first Multiplayer Internet based Boxing Game."
Free to play indefinitely.
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Outwar
An urban adventure RPG. Join a crew and defeat your enemies to gain power in Diamond City.
Free to play indefinitely.
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Overkings
OverKings is a fantasy MMORPG, played in the browser. The game is set in a mythological world, and its lore is fine, actually, revolving around a world in chaos after a struggle between two great heroes, caused by the vicious hero Nameless. There are eight character classes (warriors, healers, wizards – the usual). There are plenty of PvP and PvE opportunities as well as quests in OverKings, but most of the quests are along the lines of the usual “kill this” and “bring that”. OverKings is instance-based, and you have to explore the game world to find mobs to fight. Hint: you might prefer to avoid ‘red zones’ without a party, as other players can loot up to 30% of your equipment if you get killed.
See VideosFree, with option to pay for additional features.
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