Lots of good info and advice posted above.
Just to clarify on RTS/CTS - they are a remnant of older technology but if you need them or choose to implement them they could be useful if the interfaces you choose happen to support them. If the interfaces you choose include RTS/CTS but you decide not to use RTS/CTS you can ignore them, they will just be extra unused pins. So having them might add some flexibilty - you can use them or ignore them.
I think the more important decision is how you will manage the point to multipoint communications; from there you can decide what functions, pins, and voltages you need.
Having said that, you might be able to get the project done through hardware or software (and likely both). If you are inclined to manipulate the software and don't care too much about hardware size or cost, using an Arduino with three or however many physical ports you need might be a good solution. If you want to manage communications with hardware the RS485 approach might be a good path. The multi drop and longer wire distance supported by RS485 are two key reasons why RS485 was developed - but i'm betting that the point to multipoint requirement can be pretty readily handled in software by an Arduino (maybe a Due as suggested by ebclr). If the sensors are beyond the distance supported by RS232 you might be back to RS485. Or you might wind up with something else (ethernet, Wifi, etc). If the topology consists of short wire runs the RS232 approach should be relatively easier and less expensive.
For a quick look-see here is a TTL/RS485 converter; there are probably less and more expensive versions - but i think they will generally cost a bit more than the least expensive TTL/RS232 converters.
http://www.serialcomm.com/mobile_site/serial_rs232_converters/rs232_rs485_to_ttl_converters/rs485_to_5v_ttl_converter/rs485_to_5v_ttl.product_general_info.aspxYour oxygen sensor link says it supports both RS232 and RS485. I think for $3 more (than the non RTS/CTS board) I'd start with one of the RS232 boards that supports RTS/CTS and has the arrows. You might not need either but if either saves you any time it will be worth it. You might make a Phase I that gets one sensor successfully communicating with the Arduino and then see what it takes to add more sensors. Or you might try to design the full multi-sensor architecture from the get-go and start coding and assembling, or assembling and coding.
Sounds like a very good learning project. Let us know how it goes.