Looks good to me. I think it is hilarious. Because amnesia seems to only affect class abilities, I'm going to say the Amnesiac needs to succeed on a Will save to use its own Special Abilities and Special Attacks that would be affected by memory loss. This also changes its tactics in combat. After looking at the Amnesia rules again, the Inattentive Flaw was just overkill on my part so I nixed it. The -8 to skill penalty was too harsh when those are combined. Without feats and needing to pass a Will save to use abilities noted I'd say CR is 6. LA is — because this is not suitable for a PC.
Treant, Olive, Amnesiac
Hit Dice: 7d8+35 (66 hp)
Speed: 20 ft. (4 squares)
Armor Class: 20 (-2 size, -1 Dex, +13 natural), touch 7, flat-footed 20
Base Attack/Grapple: +5/+22
Attack: Slam +12 melee (2d6+9)
Full Attack: 2 slams +12 melee (2d6+9)
Space/Reach: 15 ft./15 ft.
Special Attacks: Animate trees, double damage against objects, spell-like abilities, slick, trample 2d6+13
Special Qualities: Damage reduction 10/slashing, low-light vision, plant traits, vulnerability to fire
Saves: Fort +10, Ref +1, Will +1
Abilities: Str 29, Dex 8, Con 21, Int 12, Wis 16, Cha 12
Skills: None due to amnesia (-4 to use any skill, see below)*
Feats: None due to amnesia
Environment: Any warm land
Organization: Solitary or grove (4-7)
Challenge Rating: 6
Alignment: Usually neutral
Advancement: 8-16 HD (Huge); 17-21 HD (Gargantuan)
Level Adjustment: —
Amnesiac olive treants are generally shorter and squatter than other treants. They have broad canopies with silvery green that do not change with the seasons, but they blossom in the spring with feathery white flowers.
An amnesiac olive treant’s legs fit together when closed to look like the gnarled and twisted trunk of a tree and a motionless treant is nearly indistinguishable from a tree. They move slowly and with an awkward gait.
An amnesiac olive treant is about 25 feet tall, with a "trunk" about 2 feet in diameter. It weighs about 4,500 pounds.
Treants speak their own language, (plus Common and Sylvan. Most also can manage a smattering of just about all other humanoid tongues—at least enough to say "Get away from my trees!" However, an amnesiac olive treant cannot remember its other languages until it hears them spoken first).
Unlike normal olive treants, amnesiac olive trees are much less likely to watch potential foes carefully before attacking. They often charge suddenly from cover to trample the despoilers of groves. Even if sorely pressed, they may forget to animate trees as reinforcements.
Animate Trees (Sp): A olive treant can animate trees within 180 feet at will, controlling up to two trees at a time. It takes 1 full round for a normal tree to uproot itself. Thereafter it moves at a speed of 10 feet and fights as a treant in all respects. Animated trees lose their ability to move if the treant that animated them is incapacitated or moves out of range. The ability is otherwise similar to liveoak (caster level 12th). Animated trees have the same vulnerability to fire that a treant has. An amnesiac olive treant must succeed on a Will save (DC 14) to use this ability.
Amnesia: Type insanity; Save Will DC 20 Onset immediate Effect –4 penalty on Will saving throws and all skill checks; loss of memory. A character suffering from amnesia cannot remember things; his name, his skills, and his past are all equal mysteries. He can build new memories, but any memories that existed before he became an amnesiac are suppressed. Worse, the amnesiac loses all class abilities, feats (including racial bonus feats), and skills ranks for as long as his amnesia lasts. In the case of creatures without class abilities but with special abilities and attacks that would be memory related, a creature has to succeed on a Will save (DC is 10 + 1/2 creature's HD + creature's charisma modifier) in order to use those abilities. Affected abilities are noted accordingly. If the amnesia is later cured, the afflicted creature regains all the full abilities of its race and class, including those gained from any levels taken while suffering from amnesia.
Double Damage against Objects (Ex): A treant or animated tree that makes a full attack against an object or structure deals double damage.
Slick (Ex): Due to use of the grease spell-like ability (not to mention residue of oil from smashed olives), the area within 30-feet of an amnesiac olive treant is slippery and hampers movement as difficult terrain. Creatures are limited to half their normal land speed. Treants and animated trees are unimpeded and can move normally.
Spell-Like Abilities (Su): At will—goodberry; 3/day—grease. An amnesiac olive treant must succeed on a Will save (DC 14) before it can use its spell-like abilities. If it fails, it cannot remember it has the ability.
Trample (Ex): Reflex DC 22 half. The save DC is Strength-based. An amnesiac olive treant must succeed on a Will save (DC 14) before it can use this ability.
Skills: *Treants have a +16 racial bonus on Hide checks made in forested areas. Due to its amnesia, the treant has no skills, but can Hide in a Forest at +3 (+16 racial, +0 ranks, -1 Dex, -8 Size, -4 amnesia) and it takes -4 penalty to use any skills.