with  coords (jsn) as (
select '{
"coordinates": [
[49.7798774,51.8828385],
[49.7794911,51.882802],
[49.7793302,51.8827822]
]
}' from dual
)
select cc.x, cc.y  from coords
, json_table(
    jsn,'$'
    COLUMNS NESTED PATH '$.coordinates[*]'
    COLUMNS (
      X number path '$[0]'
     ,Y number path '$[1]'
      )) as cc
;

         X          Y
---------- ----------
49.7798774 51.8828385
49.7794911  51.882802
49.7793302 51.8827822

with  coords (jsn) as (
select '{
"coordinates": [
[49.7798774,51.8828385],
[49.7794911,51.882802],
[49.7793302,51.8827822]
]
}' from dual
)
,  xy ( x,y) as (
select cc.x, cc.y  from coords
, json_table(
    jsn,'$'
    COLUMNS NESTED PATH '$.coordinates[*]'
    COLUMNS (
      X number path '$[0]'
     ,Y number path '$[1]'
      )) as cc
)

select MDSYS.SDO_GEOMETRY(2001,8265,SDO_POINT_TYPE(x,y,null),null,null) as obj from xy;

OBJ
------------------------------------------------------------------------------------------------------------------------------------------------------
SDO_GEOMETRY(2001, 8265, SDO_POINT_TYPE(49.7798774, 51.8828385, NULL), NULL, NULL)
SDO_GEOMETRY(2001, 8265, SDO_POINT_TYPE(49.7794911, 51.882802, NULL), NULL, NULL)
SDO_GEOMETRY(2001, 8265, SDO_POINT_TYPE(49.7793302, 51.8827822, NULL), NULL, NULL)

with  coords (jsn) as (
select '{
"coordinates": [
[1.0,1,0],
[1.0,2.0],
[2.0,2.0],
[2.0,1.0]
]
}' from dual
)
,  xy ( a, b, x,y) as (
select cc.row_number as a, cc.row_number+1 as b, cc.x, cc.y  from coords
, json_table(
    jsn,'$'
    COLUMNS 
    NESTED PATH '$.coordinates[*]'
    COLUMNS (
      row_number for ordinality
     ,X number path '$[0]'
     ,Y number path '$[1]'
      )) as cc
)
, ords(wkt) as (
select 'POLIGON(('||replace(ltrim(sys_connect_by_path(pair,'X'),'X'),'X',',')||'))' as wkt  
from (
select 
a
,x||' '||y as pair
, count(*) over () cnt
from xy
)
where a = cnt
start with a  = 1
connect by a = PRIOR a + 1
)
select 
wkt
--,SDO_GEOMETRY(wkt) as OBJ
FROM ORDS

with t(s) as (select N'POLIGON((1 1,1 2,2 2,2 1))' from dual)
select 
s
-- ,sdo_geometry(s) 
,SDO_GEOMETRY(N'POLYGON((1 1,1 2,2 2,2 1))') as o1 
from t
;