By Horav Yosef Yeshaya Braun, member of the Badatz of Crown Heights
Chazal decreed that we may not drink mashkin shezavu (liquid that flowed out of a fruit) on Shabbos, gezeirah shema yischot (as a precautionary rabbinical decree due to the concern that it may lead one to intentionally squeeze the fruit for its juice and commit the melachah of sechitah).
Although this decree only concerns olives and grapes, or other fruit normally used for juicing, it still applies even if we decide to eat that particular fruit. However, drinking the juice that oozed out of fruit which are typically omed la’achilah (intended for eating), is permitted on Shabbos.
In light of the above, since the majority of oranges today are used for producing orange juice, one would think that were an orange to be cut for eating, it would be forbidden to drink the juice that flowed out as a result. However, since the juice did not ooze out on its own, but rather as a result of cutting the orange in order to eat it, the juice is permissible regardless of what the fruit is typically used for. However, some are stringent in this case as well.