i am from Croatia and i am planing on using an special kind of an oak tree can i use the soil that is in the region it grows naturally? or do i have to use bonsi mix soil?

You don't need a special bonsai mix for any tree, although many will say they get best results that way. Most important for any potted plant is good drainage as waterlogged roots will rot. I use a peat based general purpose compost mixed 50/50 with perlite (vermiculite, horticultural sand or fine gravel would be OK). I don't think Oaks are particularly fussy regarding soil type