Looking to buy a Little Phatty, part of the reason is to interface it with my Minimoog old school.
Would the little Phatty have to have CV out in order to do this? Also, I’ve never used Midi before, but could I essentially use the Little Phatty as a CV to Midi converter for my Old school?
I’m new to synths, so bear with me. Thanks.
The LP does not have a built-in MIDI to CV converter. You would need CV outs on the LP in order to control the Voyager with the LP. 
The main reason I want one is to use my Voyager OS with the Phaty’s arppegiator. This isn’t possible?
If you have a MIDI to CV converter, or CV outs on the LP, yes, it is possible. LP’s with CV outs should still be avalable somewhere.
Or if you get a LP with cv outs, you can drive the VOS that way. (Arp notes do actuate out of the CV/gate jacks). Midi CV converters are relatively inexpensive or if you want ‘more’, you can pickup a used MP-201; it’s converter is very good since you can adjust it to fractions of a volt.