This has been hammered into my head over and over again recently. Once you get past the stage where you "know" object-oriented programming, and really begin to learn what OOP is really about (and can recognize the status quo procedures with objects), things get tough--real tough. Trying to leverage your newly learned OOP principles into day-to-day development, you will want to go into one of two directions.
Following your old style of development, your first reaction is likely to try for the Big Up-front Design. This can get messy. Trying to figure out the "proper" place to put something and deciding when to add a new object can be quite the task. If you are like me, you will also end up trying to plan for future development. You hear yourself saying, "Wouldn't it be useful down the road if.."
Stop! Hold it right there. There's a much better way. Unfortunately, it involves another shift in thinking. Test Driven Development is a great way to get started in the land of OOP. It forces you to think about the consumer of your code before you write it. It also allows you to bite off a little piece of functionality at a time. Instead of trying to build the "one object to rule them all," you end up with a much cleaner api for your object model (along with smaller, more cohesive classes). Oh, and you can tell your boss your code comes with the quality "baked in."
Another thing which can help are Class Responsibility Collaboration cards (CRC cards). Quickly jotting down the name of the class along with a one or two sentence description of it's responsibility will help answer those sticky questions on whether to add a new method to an existing object or to add a completely new object (sometimes it seems really gray and you end up twisting an older class with high cohesion into a smelly wart).
In short, while TDD and OOP are completely separate concepts, they really go hand-in-hand. It seems that learning one without the other would be quite a bit harder (at least for those of us without mentors standing behind us, correcting us as we go). So, if you are in the middle of really learning how to program properly (as I am), I strongly encourage you to give TDD a try.
[Update: Here's a great link for reading more about CRC cards if you are interested. It's the original paper written for OOPSLA written by Kent Beck and Ward Cunningham way back in 1989.]