-- Depending on connected user grants:
SELECT * FROM USER_TAB_COLS WHERE TABLE_NAME = 'my_table';    -- User tables
SELECT * FROM ALL_TAB_COLS WHERE TABLE_NAME = 'my_table';    -- Available to user
SELECT * FROM DBA_TAB_COLS WHERE TABLE_NAME = 'my_table';    -- All schemas

-- Columns from all tables in a schema (adapt with USER_..., DBA_... or ALL_...):
SELECT c.TABLE_NAME, c.DATA_TYPE FROM ALL_TAB_COLS c
JOIN ALL_TABLES t ON t.OWNER = c.OWNER
WHERE OWNER = 'my_schema';