Can I lose money on overseas bonds because of exchange rates?

Yes. Because overseas bonds pay principal and interest in foreign currency, even if you make a profit in foreign currency terms, you can incur a loss if the exchange rate has fallen when you convert back into won.

The Financial Supervisory Service has cited a case in which an investor bought Brazilian government bonds paying 10% annual interest but suffered a loss in won terms because of the fall in the Brazilian real. You should also look at factors that affect exchange rates, such as the economic situation of the issuing country.