You are correct that objects of the postpositions take the oblique form. However, घोड़ा is actually the subject of this sentence, so it's in the direct (nominative) form.
In this sentence मेज़ is actually the object of the preposition पर , so मेज़ is actually in the oblique case. But मेज़ is one of those words which has the same spelling for the singular direct and singular oblique cases, so we can only infer its case from the fact that it precedes the postposition पर .
If we swap around some words to say 'the table is on his horse', we get मेज़ उसके घोड़े पर है , and we can now more clearly see that उसका घोड़ा has converted into the oblique form उसके घोड़े because it is now the object of the postposition पर .