DC-DC Step Down converter for Solar Panel
The open circuit voltage is the voltage you must expect from the solar panel under no load (zero current draw).



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All DC-DC converters (even high efficiency ones) have a minimum input current consumption. But in most cases it'll be low enough (compared to the current capability of the solar panel) to be considered considered almost zero.

Thus, your DC-DC converter must be able to accept the full open circuit voltage at its input, just in case it doesn't have a load connected at the output. The DC-DC converter should be able to deal with the line voltage variation imposed by the panel (most of them are able to) as well.

Also, as suggested by Rohat Kılıç in the comments to your post, you should consider a DC-DC converter with an input range wider than the nominal one required, in order to avoid nasty surprises. The open circuit voltage depends on temperature and other environmental and operating factors, so taking adequate design margins will definitively be a good idea in this case.