That is basically correct. One MIDI cable connected from the MIDI Out connector of the OT to the MIDI In connector of the external synthesizer is all you need to sequence the synthesizer. Of course you will need audio cables in order to hear the output of the synthesizer.

The Nord Rack 2X has four parts or slots: you can set up a sound on each one and assign a different MIDI channel to each. If you assign those MIDI channel numbers to four MIDI sequencer tracks on the OT, you can sequence the four NR2X sounds form the Octatrack.

The Blofeld can in theory do the same but with 16 different sounds (although the OT only has 8 MIDI sequencer tracks), although many people find that the Blofeld does not cope very well with 16 sounds at once.

Think carefully about what you want and look for a good deal for your synthesizer before you buy.