There is not one, at least not in the English language.
Ostensible means something which appears as such. As in, "he was ostensibly a jerk, but in actuality he had a heart of gold". He appeared to be a jerk, but he was not a jerk.
There is no singular word as an antonym for the simplification of the opposite, that one does not appear to be a jerk, but is/is not a jerk.
Though technically not in the dictionary, you can ad a qualifying un- to the beginning, or possibly in-; though the use of un- or in- or non- depends on the origin of the word it is modifying, they mean the opposite of whatever they are attached to. This would be the only way. Words that come close to meaning the opposite (though with varying in connotation and usage) would be improbable or various synonyms of that.
However that is not an antonym. Improbably means to "appear to be unlikely", not to "not appear to be likely".
apparent -apax
"The ostensible reason for the meeting turned out to be a trick to get him to the surprise party."
Ostensible is an adjective to describe something as seeming, apparent, or appearing to be true, but not necessarily so. Example sentence: People sometimes believe media statements because of their ostensible authority.
The ostensible purpose for going over to her neighbor's house was to borrow a cup of sugar, but she really just wanted to see her friend's new furniture.
The antonym of the word 'synonym' is 'antonym'.
irrational - The antonym for "judicious" would be "irrational," meaning lacking reason or logic in decision-making. actual - The antonym for "ostensible" would be "actual," meaning real or genuine rather than appearing or stated. unlucky - The antonym for "fortuitous" would be "unlucky," meaning unfortunate or having bad luck.
apparent -apax
"The ostensible reason for the meeting turned out to be a trick to get him to the surprise party."
My father was ostensible to go to the Railway Station to pick up my grandfather.
Ostensible owner is the one who is not the real owner but is permitted by the true owner to act on his behalf.
Ostensible is an adjective to describe something as seeming, apparent, or appearing to be true, but not necessarily so. Example sentence: People sometimes believe media statements because of their ostensible authority.
immense.
An emergency room
PDQ Bach
Apparent, alleged, illusive, avowed
The ostensible purpose for going over to her neighbor's house was to borrow a cup of sugar, but she really just wanted to see her friend's new furniture.
Actual authority is the power given to someone to do a thing or carry out an assignment. it specifies which duty to do. ostensible authority is power derived from the power given to do an act. This authority is derived as a result of the actual authority given to do an act.