Structure of Ca2B6O11
1) Oxygen CAN form 3 or 4 bonds. The most common example is $\ce{H3O+}$ ion This form of bonding is critical to understand basic nature of ammonia solutions, so for exact description of it look that part of the book.

2)Boron also can form more than 3 bonds if at least one its partner provides an electron pair for bonding, not one electron.

As a general rule I prefer to stay away from the chemistry of borates and alumina-silicates as it is ruled largely by ionic interactions and non-intuitive hydration/dehydration.  

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In structural diagrams of polymeric salts 'unit' often refers to some recurring fragment... however, it is not nessesarily has same stochiometry than formal anion. In alumosilicates the anionic frame consists of $\ce{AlO4-}$ and $\ce{SiO4}$ tetrahedral units. However, they are usually connected by shared vertexes, so the exact formula of formal anion is different. Borates are a bit more complex system, since they may include both $\ce{BO3}$ and $\ce{BO4-}$ units, and oxygen is often connected to hydrogen.

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I looked into open cristallographic database. Unsurprisingly, it does NOT feature a compound $\ce{Ca2B6O11}$. However, salts are often written in 'dehydrated' form, so I looked for any borate with $\ce{Ca:B} = 1:3$ stochiometry. Several with rather complex crystal structure were found, so I left only two: one with rather simple structure of typical 'hidrated' borate anion and one with rather complex continous 'dehydrated' boron-oxygen layers. Note: the structures are refined from X-Ray , so they does not reveal hydrogen positions. Isolated oxygens of water molecules and cations are omitted. 

$\ce{B3O8}$ unit from mineral Inyoite, $\ce{CaB3O3(OH)5·4(H2O)}$

 

$\ce{[B3O6]_{\infty}}$ layer from mineral Fabianite $\ce{CaB3O5(OH)}$



and its main building block