For those BIM-illetarate, it stands for 'Building Information Model'. It is defined as  Building Information Modeling (BIM) is a digital representation of physical and functional characteristics of a facility. A BIM is a shared knowledge resource for information about a facility forming a reliable basis for decisions during its life-cycle; defined as existing from earliest conception to demolition .  For you and me; it is a software which makes life easy for constructing and later maintaining a building structure.

I am not familiar with the 'Open BIM' though. What I understand in the article is that the project did not impose a constraint on the subcontractors to use a particular brand of BIM software and let them use whatever they like.

https://www.pbctoday.co.uk/news/bim-news/open-bim-thames-tideway/71562/