You will find that all the private/expat schools in Singapore require hefty deposits. When you leave Singapore and give enough notice (eg. at least one term's notice in advance in most cases) then you get the deposit back. The *only* exceptions seem to be the sudden death of a parent! You don't get interest on the deposits either!
Did you also realise that most (if not all) schools require a one-off non-refundable registration fee too? Often around $1500-2000. Also check with your employer about the payments for the bus service - few children live within walking distance of their schools - even the local kids - and most PTA's operate a private bus transportation door-to-door service. It is very expensive, about $350 - 500 per term depending on distance, although fees are much lower for local kids using the same buses!
Please note that OFS is a profit-making school and directors/share holders take their cut of the profits. A few years back the Head teacher & many other teachers resigned over the profiteering and the (British) Headmistress later started up the Chatsworth International school. The only non-profit schools that I'm aware of where all income is ploughed into education are the Singapore American School, United World College of SE Asia and Tanglin Trust School (hence its name). They are all excellent schools.
I would suggest that you contact the schools I've just listed (all on the internet) for their termly (semester)fees, plus deposit & registration fees. Charges vary according to the age of the child, older children incur higher fees. Also check how many terms/semesters there are in each academic year, in the British system it follows a three-term year (Tanglin and UWC definitely do), I'm not sure about the American & Australian schools. Here are some expat schools you could try approaching, they should all have internet sites. Also ask how long their wait-list is for the age of your child:
Singapore American School,
Tanglin Trust School,
United World College of South East Asia,
Chatsworth Int. School,
Dover Court School,
International School of Singapore (ISS),
Canadian School,
Don't know your nationality but there are also schools for the French, German, Swiss, Dutch, Japanese and I think the Korean communities.