Hi Liv,

Tree structure of projects

Yes. Projects can be children of other projects, sharing versions, members, etc. You can have finer control through cross-project relations between work packages independently of their hierarchy.

Users in different levels: company a does not see company b

Yes. Global administrators users will always see everything in the instance, but you can actively control which users can control which projects. If you create two separate projects, managers of that project will be able to create subprojects / manage their members etc.

It only becomes more complicated if you need a company to create their own (work package types, custom fields, etc.), since they are configured globally and require administrative access. If you have a global super admin that can control these settings though, you should be fine.

time sheet / agenda of employers: when they take holiday etc

No. OpenProject has cost and time management features, but they are all targeted at time and cost estimations and the actual logging of time and costs.
However, resource management including workdays and available units of work is a whole block feature that we’re very interested in.

We do not have it on the immediate agenda, though. If you are interested in either working on such a feature as part of a plugin or help us fund the feature, please contact us. :-)

task dependency mix: one time there is dependency, one time you can switch it off

Not sure if I understood you correctly, but you can define a number of relations / dependencies of other Work Packages / Types for each individual work package.

These dependency types are parent/children, follows/precedes, duplicate/blocking. With the exception of the parent relationship, all these relations have 1:N arity.

mobile version

(No). The number of features and resulting visual complexity of OpenProject are often difficult to present in a useful manner on mobile screens. We do have a specific mobile view for viewing single work packages that are targeted for users who click on work package update notification mails on their mobile.
We do plan to provide mobile-friendly views where possible, but using the whole application on your mobile is certainly more difficult.

approval function

(Yes): If someone creates a work package and assigns you, a very usual flow would be to reject or accept it assigning it a specific status.
There is no specific whitelisting new work packages, though.

With our workflow features, you could force specific users (identified by their permissions/role in the system) to create work packages only with a specific status (e.g., Waiting for approval), and create another status that only you can change (Approved). The system is highly flexible on that end and you should be able to tailor it to that need.

Best,
Oliver