Hi,
You can have the both OS on your disk, if you want you can have up to four different Os on your disk.
First at all install the Open Linux but take care that you do not write the master boot record, after that , install the Openserver and make the openserver the active partition.
When you boot the open server and get the Boot message type:
boot: bootos # , where # is the partition number in which you install the Linux.
This is one of the possible solutions and it is easy.