The deal is the biggest building contract signed since the Covid-19 outbreak.
Sisk will begin construction on site immediately with the programme to last 30 months.
Specialist contractor Byrne Bros will deliver the £41m piling, reinforced concrete frame and groundworks packages.
Under the agreement, Sisk will deliver the project fully fitted to become home for 6,000 staff.
The office is an environmentally sustainable building designed to achieve BREAM Excellent and WELL ‘Gold’ Certification, a measure of the building’s impact on health and wellbeing of its occupants.
The campus will include, a fitness centre, cycle storage, and an employee rooftop walking and running track.
Ajaz Shafi, Director, UK South, John Sisk & Son said: “This is a landmark project comprising of circa 500,000 square foot of ultra-modern office space constructed over an expansive double basement.”
James Osborne, Osborne+Co, said: “I am delighted to have reached commercial close with Sisk on this prestigious and highly challenging project.
“Osborne+Co and Sisk have worked tirelessly together for the past several months to reach this point.
“Throughout the bid and negotiation process, the Sisk team continuously strove to find ways to deliver more economically and improve buildability without compromising on the final quality of what we will deliver.
“I have been particularly impressed with the respect and partnership the Sisk team foster with the design team and the construction supply chain.”
The client team includes WSP as building services engineer and Turner & Townsend as project manager and cost consultant.