Players in Red Dead Online have many ways to test their mettle against one another, be it alone or in a Posse.

Teaming up with your mates and riding around the Wild West as a gang provides a true frontier experience. Squaring off against other Posses can be violent, or it can be a friendly contest.
These challenges are contests that task Posses with achieving victory over others through teamwork and experience in different activities, some of which exist as solo challenges as well. You can expect XP, Gold and RDO$ payouts for winning Posse Versus Challenges.
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Biggest Fish Contest
The Biggest Fish Contest is a team version of the Fishing Free Roam Challenge. Your posse needs to catch the greatest amount of fish by weight within the time limit to win.
Make sure your posse-members are all familiar with the fishing mini-game, have had some practice, and are equipped with lures and bait. When the challenge starts, spread out to the best fishing locations instead of bunching together at the same lake or river.
Keep in mind that weight, not the number of fish, is what determines the winner in this contest so catching fewer, heavier fish is almost certainly a better way to gain an advantage than racking up tiny catches.
Bird Shooting Contest
For the Bird Shooting Contest challenge we recommend using long-range weapons, preferably equipped with a scope. Like with the Fishing contest, we also suggest spreading out as much as you can as well, because if everyone hunts birds in the same general area you'll run out of game to shoot very quickly.
Make sure to stay out of urban areas, and don't stray too close to roads as birds will be rarer in these areas.
Herb Picking Contest
There aren't many tips for the Herb Picking Contest, and what little advice we have is similar to what we've mentioned before - spread out and stay away from towns. There really isn't a concrete strategy to follow here.
Infighting
Infighting is a basic free-for-all deathmatch, played against other Posse members. There really isn't anything special going on here, it's just a standard deathmatch game mode, so there aren't any specific tips we can give you. Stay alive, aim well, and let the others weaken each other so you'll have an easier job cleaning them up.
Team Infighting
This is just the Team Deathmatch variant of the previous challenge. Make sure to communicate with your team mates, keep an eye on your positions and try to anticipate the movements of your enemies.
Posse Race
Another, fairly straight forward mode. You race against other members of the Posse, without any further modifications to the game mode rules.
Hunt the Leader
In Hunt The Leader, Posse members need to look for and defeat the Posse Leader in an asymmetrical multiplayer mode. As the leader, you need to keep a low profile, stay on the move, and try to scope out where the hunters are. As the hunters, you need to follow a search pattern and stay in contact with your allies.