Probably not possible - JN radios recognize FSK, most simple weather modules send using OOK / ASK. See these links for more info: http://talk.jeelabs.net/topic/66 and http://news.jeelabs.org/2009/12/25/ook-reception-with-rfm12b/
Also, for 433 MHz, you probably need a 17 cm antenna, and even then a 868/915 MHz RFM12B on a JeeNode may not have a very high sensitivity (i.e. range). A stand-alone cheap 433 receiver such as shown here would be better: http://news.jeelabs.org/2009/03/10/decoding-433-mhz-kaku-signals/ - and then the JN can continue to be used at the same time for 868 MHz comms.
Would be interested to find the specs for Oregon protocol. I've got one such sensor here, but I never got around to recording the 433 MHz OOK pattern, let alone figure out the codes.