Because Ooblexes are higly intellegent and capable of acurately impersonating anyone, you have inherited a sliver of these traits. Your Charisma score increases by one and your intelligence score increases by two.

You smell lightly of sulfur. Any creature can make a perception check based on smell to notice. the DC equals 10 + the number of feet the creature is from you (minimum 11). Because this smell emanates from you, you don't notice this odor yourself.

You have an extreme fear and hatred of fire, and you probably have no idea why. If you take fire damage you have disadvantage on attack rolls and ability checks until the end of your next turn.

You are made of the same slime like, sulfurous material that an ooblex is made of. You are amorphous and can move through a space as small as 1 inch wide without squeezing.

If you don't know that you are the simulacrum of an ooblex, you probably don't know that you are amorphous either. You may find out that you are amorphous if something tries to crush you, for example. Learning that you are amorphous prior to learning about your history will likely be very troubling and can very easily become a whole campaign for your party to find out what you really are.

Your unarmed strikes are infused with psychic energy. Whenever you hit with an unarmed strike, you deal 1d4 psychic damage in addition to the normal bludgeoning damage for unarmed strikes.

The psychic damage works in such a way that a creature thinks you have hit harder than you actually have. As such, you probably think that you are better with your fist than you actually are, especially if you have not yet learned of your nature.

Ooblexes grow and age by devouring the minds of various individuals, learning everything they know. As a result you know Common, one random standard language and one random exotic language. These languages should be chosen at random. If your original race knew specific languages (such as an elf knowing Elvish), you may substitute this/these language(s) for a random choice.

Ooblexes learn all the skills of those they devour. As a result, you have inherited three random proficiencies; one skill, one tool, and one weapon. These shoud be chosen randomly from among proficiencies not gained from your class or background.

Because you were created and are still sustained by powerful magic, when you are inside an anti-magic field you collapse into a gelatinous puddle and are incapacitated. When you emerge from the anti-magic field you nearly instantaneously reform into your normal form and have no memory of the event. For you there was merely a time skip, which you may or may not have noticed, depending on how much time has passed and how much your surroundings changed in that time.