If the update was to the ROM, flashing a new ROM will replace the entire old ROM, including the update.
If the update was to an app the ROM doesn't affect, it won't affect the update.
If the ROM was done correctly it'll work, otherwise it won't. (The ROM you're going to flash replaces the ROM LG updated with the OTA update, assuming that the update was to the ROM not to some app.)
"OTA" doesn't say what you got, just how you got it.