Advancements
When Freedonia updated to 1.12.2, the ability to add custom advancements was added. MinecraftOnline has a decent number of custom advancements which you can obtain in the ways listed on this page.
If you had any achievements from 1.7 or before, those are replaced with this system.
SPOILERS AHEAD - Do not continue reading this article unless you want to know how to obtain each advancement.
Easy advancements
MinecraftOnline
This one's quite easy: all you have to do is log in to the server so everyone should have it, except those who have not joined in on 1.12.2 yet.
Read The Rules
Read the rules at the Rules Hall. Pretty self-explanatory and something you should do anyway.
Spawn
A piece of cake! Go to spawn, which you get sent to right after you join the server.
A place to build
Do /wilderness, which should take you to undeveloped land.
Enter the Nexus
Travel to Nexus Central.
Visit the farm
Go to the warp farm. Many newer players do this to get food.
Not as nice as it sounds
Travel to the warp sauna, which many new players are tricked into doing despite a tip saying not to.
Free Loot
Finish the maze by finding the chest room, which has gear from poor saps who melted in the sauna.
The Freedonia Tour
Finish the Freedonia Tour, which has 17 unique locations from 2010 to 2022! Caution: Going through the tour too fast may not count towards the advancement.
The Historical Tour
Explore the Historical Tour, featuring 12 locations from 2010-2013.
The Extended Tour
Explore the Extended Tour, featuring 11 locations from 2014-2022.
I voted
Vote for MinecraftOnline to get tokens which you can spend on kits. You may vote at the server's Planet Minecraft and Minecraft Server List pages.
Teleport Taxi
/follow someone's Teleport Trails.
Re-teleporter
Use the command /follow to try and follow your own Teleport Trail.
Lost Follower
Try to follow someone's private teleport. This could be something like their home or a private alias.
Going Nowhere
Teleport to where you exactly are currently.
Liked
Receive positive reputation via someone using /goodrep on you, which also gives you a delicious Reputation Cookie. Nom nom nom.
Oh, a cookie!
Eat a Reputation Cookie, whether yours or someone else's.
Teleport Bread
Get Teleported Bread by teleporting home or to any warp with bread in your inventory.
Yummy Bread
Eat Teleported Bread.
Home Sweet Home
Use the /sethome command to save your home location. You can now return with /home!
Where am I?
Use /map.
Goodnight Freedonia
Use the command /goodnight or /gn at night.
Out of Time
Use /goodnight or /gn when in the end or nether.
Researcher
Use /nearestplace to find the closest (documented) place to you.
Documenter
Contribute to our wiki. Check the tutorial, and try starting with your own user page!
Medium difficulty advancements
Mutate Bread
Teleport with Teleported Bread and it might completely mutate.
Explorer
Visit every current warp which you can list with /warplist. Made harder by the cooldown (feel free to explore while waiting it out) and needing to keep track of where you've been.
Visit each Nexus hub on MinecraftOnline. This includes the central nexus, hubs and branches which means you have to go to 33 locations!
Into the Borderlands
Reach every Nexus Borderland Hub on MinecraftOnline. Not as hard as Navigator, but there are still 24 locations you have to go to.
Hell on Earth
You have to know the location of the nether recreation on the map in order to get this advancement.
The Old End Entrance
Find one of the original end portals on MinecraftOnline.
Teleporter Malfunction
Reach to Nexus Unknown. See around the map or search the wiki for clues.
Dungeon Destroyer
Go to a server dungeon and fight the mobs! The easiest way to get this is by activating the Shulker Ranch (or being there when someone else does).
Far Far Away
Teleport at least 35,000 m. This was very hard before the 1.12.2 expansion, but the easiest way to get is going to the other end of the expansion, setting home and then going to a warp or somewhere else on the map that is at least 35,000m away from the location.
Repopulation
You can get this by first getting a spawner by breaking a natural one or from hourly gifts, and then placing it and using the command /setspawner to set the mob you want it to spawn.
Hard advancements
Popular Destination
Get multiple people to follow your trails. A warp trail may be useful for trail travelers.
Historian
Hardest of the bunch. You must go to every area that had a warp at some point. This is difficult because you can't just /warp to the area, so you will have to get there by other travel means, like elytra, boat or plain old walking. Browsing the wiki helps a lot.
The End of The Rainbow
Kill every colour of shulker at the Shulker Ranch. This is hard because some colours may spawn at the airships, which could kill you by the shulker knocking you into the void, if they spawn at all.
Reaching New Heights
Build over y 128. This requires you to either spend 100 hours ingame or get 20 reputations, which trolling makes difficult.
Spawn Seekrits
Find the 12 seekrits around Spawn. This was part of the Easter Egg Hunt 2022 event, and each clue can be found on its page.
Bug Catcher
Find a bug in the game and report it on the issue tracker. Your bug report must be approved to complete this advancement and receive a Bug Net.
Pigchinko
Participate in a game of Pigchinko. Pigchinko events are typically held when there is a large amount of players on the server.
Cake Day
Join the server on the day you first joined. Use /firstseen to find that date.
Celebration Cake
Get a custom MinecraftOnline birthday cake given on the 4th of August.
Trick or Treat
Get a custom MinecraftOnline spooky pumpkin given on Halloween.
Merry Christmas
Get a custom MinecraftOnline Christmas tree from Christmas.
Bee Happy
Get a bee or angry bee head from various events.
Very hard/impossible (at this time) advancements
Pigchinko Arena Tour
While many arenas are easy to find/get to, the one in the end and some scarcely documented ones are difficult to reach/find. With the End being reset for 1.12.2, this means that the end arena was removed, but you can still get the advancement by flying to the coordinates listed on its old page, which leads to a high chance of death since you would have to fly with elytra in open end void, unless you are Aether donor/kit or above.
The Creator
To get this advancement you must be ingame when the owner 
SlowRiot logs in. He rarely logs in due to being busy with stuff in real life. His last login was on August 4th, 2023, for the MCO birthday.
Boat Race
Participating in Boat Race events. Stay tuned on Discord for upcoming events!
Gone Fishing
Participate in the Holidonia event which made this advancement obtainable from July 8th, 2021 to July 18th, 2021.
Under Maintenace
You could get this by participating in the November 30th, 2021 downtime or December 9th, 2021 Log4Shell temporary shut down.