Here is the full list of all 20 Old School RuneScape achievements. It takes around 0-1 hour to unlock all of the achievements on Windows.
Complete the Dragon Slayer quest
Complete a Clue Scroll
Visit Death's Domain
Craft some Air Runes at the Air Altar
Catch a Tuna at Musa Point
Visit the General Stores in Lumbridge, Falador, Varrock, Edgeville, Rimmington, Al-kharid and Karamja (Musa point)
Equip Fancy Boots or Fighting Boots from Stronghold of Security
Enter the Wilderness
Pass through the Al-Kharid/Lumbridge gate without paying
Complete the Demon Slayer Quest
Use the Home Teleport Spell with any spellbook
Complete the Vampyre Slayer quest
Chat clothing with Thessalia in Varrock
Ride the canoe in Lumbridge
Reach Combat level 20
Equip a Mithril Scimitar
Reach a total level of 100
Complete the Cook's Assistant quest
Complete the Tutorial
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