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
Pass through the Al-Kharid/Lumbridge gate without paying
Complete the Vampyre Slayer quest
Ride the canoe in Lumbridge
Complete a Clue Scroll
Visit the General Stores in Lumbridge, Falador, Varrock, Edgeville, Rimmington, Al-kharid and Karamja (Musa point)
Equip a Mithril Scimitar
Complete the Demon Slayer Quest
Craft some Air Runes at the Air Altar
Catch a Tuna at Musa Point
Chat clothing with Thessalia in Varrock
Complete the Tutorial
Use the Home Teleport Spell with any spellbook
Equip Fancy Boots or Fighting Boots from Stronghold of Security
Reach a total level of 100
Visit Death's Domain
Enter the Wilderness
Vote in a Poll or view Poll results
Reach Combat level 20
Complete the Cook's Assistant quest