Uh... generally, OEMS come with almost no warranty while retail get up to 3+ years. uh... retail is more expensive.. regarding pentium3, oem is generally the older kinds of stepping processors, while retail would have the cc0 stepping. im not exactly sure. all i know is oem is cheaper.
edit: cc0 is the newest stepping version out by p3, best for overclocking. oem may not get this.