[wups]

Change

pawn Code:

for(new i = 0; i < CountDynamicObjects(); i++)

to

pawn Code:

for(new i = 0,j = CountDynamicObjects(); ; i < j; i++)

This way you gain efficiency.

Variable names are very common

pawn Code:

new
        Float:px,
        Float:py,
        Float:pz,
        Float:ox,
        Float:oy,
        Float:oz,
        Float:distance = MAX_DISTANCE
;

You should consider something less common, and use them locally. You don't need them global.
+ distance is global. Let's say I call GetClosest(playerid,OBJECT) and it is 2.0 units away. Okay, the next time I call the function, it won't check for anything further than 2.0 units! Test your code next time.

pawn Code:

new Float:odist = floatsqroot(
            floatpower(floatabs(floatsub(ox, px)), 2.0) +
            floatpower(floatabs(floatsub(oy, py)), 2.0) +
            floatpower(floatabs(floatsub(oz, pz)), 2.0)
        );

Why all this?

pawn Code:

dist = GetPlayerDistanceFromPoint(playerid,x,y,z);

pawn Code:

stock GetNearest_Object(playerid)
{
        new currentobject = -1;
    GetPlayerPos(playerid, px, py, pz);
    for(new i = 0; i < MAX_OBJECTS; i++)
    {
                if(!IsValidObject(i)) continue;
        GetDynamicObjectPos(i, ox, oy, oz);

It's not GetDynamicObjectPos.

In conclusion: You should look in to the functions, before releasing them. Try to study, look for better methods.
Cheers.