If you want to simply extend a L2 VLAN, with no SVI enabled in that VLAN, nobody stops you to connect your servers directly to the OTV VDC.
However, the limitations comes to SVI + OTV VDC combined. This VDC cannot perform routing for your SVIs and also perform the encapsulation with OTV header. This is why most (if not all) of the deployments have a VDC where the SVIs (server's gateway) is configured, and another VDC strictly for the OTV encapsulation.
There are multiple designs for how to insert OTV in your network: OTV on a stick, where both join and internal interface are connected to Core VDC, or in-line design, where join interface is connected to a different device (the router part of L3 network between DCs) and the internal interface connected to CORE VDC.
For more details about OTV design, my suggestion is to read this whitepaper: http://www.cisco.com/en/US/docs/solutions/Enterprise/Data_Center/DCI/whitepaper/DCI3_OTV_Intro_WP.pdf
On the other hand, if you are interested in technologies for DCI, I would suggest to read about vxlan multi-site or ACI (multi-pod/multi-site). It is a much more up-to-date technology and VXLAN is definitely a more scalable protocol.
Stay safe,
Sergiu
