Object layer in the background

is it possible to put a object layer in the background? i want a tile layer to overlap the main character.

Yes, just put the object layer below the tile layer. You can control the stacking order of the layers using the actions in the Layer menu or using the small buttons below the layer list.

in the map editor it overlaps, but when i execute it in the engine the object layer is always on the foreground

This indicates a problem with the engine you’re using, so please report it there. What engine are you using?