This what a terminal server component is. In general some number of RS-232/Serial ports (today 9 pin connector) and at least one Ethernet LAN interface:
http://www.moxa.com/product/Terminal_Servers.htm
You then connect the LAN interface to some Ethernet network and access the various terminal servers remotely. For true OOB you need a separate OOB network for this. Some folks do this by connecting to a LAN port on the local switch, but when the switch may have a problem, defeats the purpose being able to get to the switch remotely, no matter what.
Other remote mgmt option is using some in-band IP address on the switch(es) themselves. For more on terminal servers go here:
https://technet.microsoft.com/en-us/library/cc755399%28v=ws.10%29.aspx
You can find lots of information about this subject with simple Google/Yahoo/whatever search.