OpenShift has implemented a scalable design for it's hosted Jenkins service that spins up new Jenkins worker environments in order to processes each build.
This design requires you to have an extra OpenShift gear instance available when a given build task is run. If your account has sufficient capacity, then you should be able to run several builds in parallel.
If you want to use Jenkins on OpenShift's Free plan, then I'd try keeping one spare gear available at all times to make sure that Jenkins is able to keep working.
For users who have upgraded to one of OpenShift's paid plans: OpenShift does not charge hourly fees for the additional resources that are used while running your Jenkins workers / build tasks. (no hourly fees for running a build, since it takes less than an hour)