The first one means that you will be at the office at 9 O'clock. The second one means that you will be on your way to the office by 9 O'clock which means you will arrive after this time.

A few unrelated language points first: There is no such word as 'tmrw'. This is a meaningless sequence of letters. You can't say 'go to office'. 'Office' is a singular countable noun, so it needs an article. Now to your question: 1. We mostly use the 'will' form when you are making a promise/offer, or making a decision at that moment. For example, in a phone call the night before from your boss: 'We need someone to be at the office to meet the delegates tomorrow morning. They're arriving at 9.30, but I can't be there.' 'OK. I'll go to the office around 9 o'clock tomorrow morning.' 2. We use the future continuous to say what will be happening, or is scheduled to be happening, a particular time in the future. For example, your sister might say to you, at some point after the above phone call: 'Can you give me a lift into town tomorrow?' 'Well, I'll be going to the office around 9 o'clock tomorrow morning. You can come with me then if you want.'

Just some corrections: Say that you'll go to THE office or you'll be going to THE office etc etc... I'll go to the office around 9 o'clock tomorrow morning I'll be going to the office around 9 o'clock tomorrow morning In the above context, there is no difference in the meaning. HOWEVER, if you did not specify a time, "I'll go to the office" implies that you are going to the office with an emphasis on the near future. "I'll be going to the office" does not offer any sort of time, only the fact you will go to the office at some point.