The default software synthesizer in One Man Band is JmSynth, a low latency version of the Microsoft synthesizer (Quicktime synth on the Mac). There are many other software synthesizers available on the market.
Most commercial synthesizers can be used stand-alone, to connect such a synthesizer to OMB you must use a virtual MIDI cable like e.g. LoopBe1 for Windows or IAC that is part of OSX (Application>Utilities>Audio MIDI Setup>MIDI Devices). Once installed the virtual MIDI cable can be selected in the Preferences window.
Using VST plugin synthesizers with One Man Band for Windows
Specify up to 4 different VST instruments in the Preferences window (MIDI Out tab). These instruments will become available in the MIDI Out list. The userinterface/panel of each VSTi will become available in the WIndows menu in OMB. Not all VST instruments will work in OMB but there are a lot of free instruments available that work fine: www.vst4free.com
Make the buffersize shorter to reduce latency, make it longer to avoid distortion. If you have an ASIO driver installed (e.g. Asio4All) then you may be able to get a better latency result. But be aware that once ASIO is activated in OMB, you are no longer able to produce any sounds on your PC that goes thru the normal sound drivers, even jmsynth will no longer work.
Using Soundfonts in One Man Band for Windows
Soundfonts are wavetables with instrumentsounds that you can play thru a soundfont player. One of the free SF players is CoolSoft VirtualMidiSynth.
Download and install VirtualMIDISynth there are soundfonts there too if you need any. After this it will become available as an extra MIDI output port in One Man Band.
When you select the Coolsoft VirtualMIDISynth in One Man Band for MIDI output, its configuration window icon will become visible in Windows status bar. In the configuration window add a sountfontfile in the Soundfonts tab and set the additional output buffer to 20 in the Advanced tab.
Now restart Coolsoft VirtualMIDISynth by restarting OMB.
You can now play the soundfont from within One Man Band.
Using DXi synthesizers in One Man Band for Windows
Synthie will enable you to play a DXi synthesizer like Coyote's Forte DXi or the Virtual Sound Canvas.
the One Man Band main page.