Quoting from the SIBOR wikipedia entry,
SIBOR stands for Singapore Interbank Offered Rate and is a daily reference rate based on the interest rates at which banks offer to lend unsecured funds to other banks in the Singapore wholesale money market (or interbank market).
On the other hand,
The Swap Offer Rate (SOR) represents the effective cost of borrowing SGD synthetically through borrowing USD for 3 months and swap out the USD in return for SGD for the same maturity.
The fixing authority for both rates is the Association of Banks in Singapore. To summarize some of the differences between the two:
- Sibor is influenced by the supply and demand for funds in the Singapore interbank market, whereas SOR is more exposed to factors external to Singapore such as USD interest and exchange rates.
- SOR is more volatile because exchange rates as well as USD money market rates tend to fluctuate more