How about a water feature to mask the sound the other option is a triple glazed window.

Hi Patricia, unfortunately the triple glaze window is out of the question as my house is almost brand new... Water feature sounds nice. thanks for the suggestion :-)

We lived on a noisy street once and kept a window AC unit running full time to provide "white noise". In the winter of course we just had the "fan" on but that's what makes all the white noise so it worked great.

Upholster the walls in the office - it will help with the sound and looks great.

Landscaping might help. Tall trees/shrubs can soften noise, but they take awhile to grow in.

Softer (upholstered) furniture near that wall will help too.

Perhaps a type of bamboo? Quick growing, but can be invasive.
Podacarpus also would work. Get 15-gallon size.
Possibly a privacy screen in front of the window with a vine growing on it? That would deflect some noise as well as being attractive. Houzz.com has some interesting pix of such ideas.

Wow, cool, you all have fantastic suggestions! I already added the curtains and it works well: it reduces quite a lot of noise actually... Now for bamboo, I live in a zone 5 and I'm afraid bamboo will not do good in my area and my neighbors will probably hate it as they are grass lovers and admirers...