My parents want to donate an old oven/range from a second kitchen in their home to their church for use during the annual festival. So, based on that, I know that the oven is 240v single phase.

Our disconnects for the festival are all 3phase, wired to temporary panels. Am I correct in assuming that the oven cannot be used because of the voltage difference? (not to mention the code issues of running an indoor appliance outdoors)

If you plug a 240 volt oven into a 208 volt outlet, the consumption of electricity will be as follows:

Power consumed = V^2/R

At 240 volts this will be 57600/R
At 208 volts this will be 43264/R
where R is the resistance of the heating element.

The ratio of power consumed at 240 volts vs 208 volts is

57600/43264 = 1.33

Basically, you use one-third more power at 240 volts than 208 volts. Or, put another way, you use 75% of the power at 208 volts that you would at 240 volts.

This isn't particularly dangerous since it's using only resistive heating elements. The oven simply won't get as hot as it might since you're not dissipating as much heat. However, if the insulation is good enough, it will probably work. It just won't heat as fast.

I can't comment on the code issues.

Most residential ovens are actually rated for 240v or 208v. Look at the rating plate, usually inside the oven door.