Java Edition:Oak Leaves (Item)
Oak Leaves were not supposed to be obtainable until shears were introduced in Beta 1.7.
Obtaining
Indev Item Chests
From Indev 20100124-1 to Indev 20100128 new worlds would generate with the Indev House, and inside the house were debug chests. Between Indev 20100124 and Indev 20100125 the chests held items in stacks of 99, while in Indev 20100128 the chests held items in stacks of 100. Oak leaves were one of the such items that generated inside the debug chests.
Beta 1.6 Test Build 3 Debug Chests
In Beta 1.6 Test Build 3 new worlds would generate with 11 debug chests. Within the debug chests were a wide variety of items most of which were available in there full stacks some were not. Oak leaves were one of the such items that generated inside the debug chests, oak leaves generated in the 1st and 5th chests from the left in the 18th and 2nd slots respectively, in stacks of 64.
1st chest from the left.
5th chest from the left.
Mining
From Beta 1.7 until 24w20a (present) oak leaves could be collected using shears. Additionally oak leaves can be obtained using Silk Touch from its addition in Beta 1.9 Pre-Release 4 until 24w20a (present), when mining oak leaves with a Silk Touch tool or shears the block would drop itself instead of nothing. Silk Touch does not work in 12w32a (1.4.2 Snapshot) due to a bug, causing the Silk Touch method to be discontinued for that snapshot.