Is there a way to download OZ Mobile/T-Mobile IM onto the computer? If so, where can I find a copy? Thanks.

Get the OTA copy pushed to your device (usually can be accomplished by turning it on) and use javaloader to save the modules to your PC.

The thing is I can't get it OTA since I don't have a data plan. I'm trying to figure out another way to obtain it.

Edit: Forget this... I'm getting the BIS Add-On. Problem solved.

Oz uses SMS tho, which means you'll be charged for every SMS.
Even if you have an unlimited SMS plan messages are delivered much slower over SMS than over GPRS, so its much better to use something like Verichat or IM+ or one of the free ones.