The most robust method would be to use Oracle Goldengate but that might be overkill for your requirement. (It is also a licenseable product)

http://www.oracle.com/technetwork/middleware/goldengate/overview/index.html

The other option is hand-coding the transfer. First you need to setup heterogeneous services

https://asktom.oracle.com/pls/apex/asktom.search?tag=connectivity-to-sql-server

but in a nutshell
- setup a database link called (say) "dblink"
- this points to a tns entry called "sqlserv"
- in $ORACLE_HOME/hs/admin you initSQLSERV.ora
- this points to an ODBC entry which connects to your SQL Server database

And then you'll be using SQL to copy the data over. SQL Server has case-sensitive names, so typically queries will look like:

insert into my_oracle_table
  (COL1,
   COL2,
   ...
  )
  select 
     rtrim("Col1"),
     rtrim("Col2"),
     ...
  from MY_SQL_SERVER_TABLE@dblink;