Cicero provides us with a lot of personal detail in his letters and he also spends time discussing his various holdings (i.e. properties, including his country villa in Tusculum) and his daily work flow.

You can find extensive reflections by the wealthy and influential Cicero in his letters to Atticus and to his brother Quintus. These reflections are mixed in with general philosophizing and commentary on the politics of the day (rise of G.I. Caesar). It's all quite fascinating and worth a read, if you ever find the time and energy.

To Atticus in particular Cicero directed his questions regarding aesthetics and taste; he relied on his friend for, essentially, fashion advice. When Cicero was later on in age, he became quite preoccupied with outfitting his villa and making it conducive to his needs, as he was beginning to prepare for his political retirement (sadly, he got killed). These preparations included the mass acquisition of furnishings, artwork, garden accoutrements and also the establishment of a library suitable for his writing needs.

So, to the meat of your question:

The first thing to understand is that Romans did not generally write. They dictated, and this is one of the reasons why, when you read ancient Roman texts, they are far more conversational than the stilted, formalized academic writing that are commonly associate today with "educated discourse". You should envision two people in this "office", namely the master and the slave, one dictating, the other writing. This is in contrast with today's offices where (at least, in my profession) the dictations are recorded, and then transcribed in another room. I should note that even 30-40 years ago, it was common for attorneys to call their secretaries into their personal offices to "take a dictation" as well.

What I say now is an exception to the above rule:

Cicero describes how he wrote a "short note" to his friend Atticus on a tablet and gave it to Eros, a messenger slave ("Quae decideras, omnia scripsi in codicillis eosque Eroti dedi"). So dictation was not always used, especially not for short, brief notes. The tablet would have been wax, the inscriptions on it would have been made by a stylus. Any Roman worth his salt would have had a ready supply of runners and messengers at his disposal, as well, in order to ensure the swift conduct of his communiques (this would include a stable of horses at a country estate; in the urban settings, I'm not sure whether this would have also been true).

In other words, you would have seen a supply of wax tablets and styli in a given Roman office next to a writing desk for the scribe and the usual writing utensils (at this time, parchment, ink).

What else would have been in the office?

If you were Cicero, lots of statutes - a popular style of art.

We read in Cicero's letter to Marcus Fadius Gallus:

"But you, being unacquainted with my habits, have bought four or five of your selection at a price at which I do not value any statues in the world. You compare your Bacchae with Metellus' Muses. Where is the likeness? To begin with, I should never have considered the Muses worth all that money, and I think all the Muses would have approved my judgment: still, it would have been appropriate to a library, and in harmony with my pursuits. But Bacchae! What place is there in my house for them? But you will say, they are pretty. I know them very well and have often seen them. I would have commissioned you definitely in the case of statues known to me, if I had decided on them. The sort of statues that I am accustomed to buy are such as may adorn a place in a palaestra after the fashion of gymnasia. What, again, have I, the promoter of peace, to do with a statue of Mars? I am glad there was not a statue of Saturn also: for I should have thought these two statues had brought me debt! I should have preferred some representation of Mercury: I might then, I suppose, have made a more favourable bargain with Arrianus. You say you meant the table - stand for yourself; well, if you like it, keep it. But if you have changed your mind I will, of course, have it. For the money you have laid out, indeed, I would rather have purchased a place of call at Tarracina, to prevent my being always a burden on my host. Altogether I perceive that the fault is with my freedman, whom I had distinctly commissioned to purchase certain definite things, and also with Iunius, whom I think you know, an intimate friend of Avianius."

Romans also loved paintings (oil based) and frescoes, same source:

"I have constructed some new sitting-rooms in a miniature colonnade on my Tusculan property. I want to ornament them with pictures: for if I take pleasure in anything of that sort it is in painting."

Any other questions about Roman interior design?