Sorry if I'm slightly wrong on this did the IMC a long long time ago, but before the closing auction price which is the EOD close there is the opening auction price.
The code is OFFICIAL_OPEN_AUCTION_PRICE or PQ814
Tip for searching in Bloomberg when you do get to use one is to load up the security you require and then type FLDS, then use this to search for what your after, the FLDS screen shows all Fields available for the ticker entered, and if they are historical or realtime and the name of the field.
PQ814 - Official Open Auction Price (OFFICIAL_OPEN_AUCTION_PRICE)
Official open auction price represents the price at which at which all orders had been
executed upon conclusion of the opening auction call trading phase.
The associated executed volume is available in the accompanying field Official Open Auction
Volume (PQ815, OFFICIAL_OPEN_AUCTION_VOLUME) and Official Open Auction Volume Realtime
Only the opening auction call is represented by this field. For other auction calls, the Official
Auction Price (PQ049, PX_OFFICIAL_AUCTION) and Official Auction Price (Realtime) (RQ443,
PX_OFFICIAL_AUCTION_RT) field (for all types of auction call) and Official Close Auction Price
(PQ816, OFFICIAL_CLOSE_AUCTION_PRICE) and Official Close Auction Price Realtime (RQ816,
OFFICIAL_CLOSE_AUCTION_PRICE_RT) (for the closing auction call) are available.
The price and size at which orders would be executed were the auction call to conclude at the
current time, otherwise known as the indicative or theoretical uncrossing price/size, are
available during the auction call in Theoretical Price
(PR089, PX_THEO) and Theoretical Price (RQ089, THEO_PRICE) and Theoretical Volume (PR160, PX_VOLUME_THEO) and Theoretical Volume (RQ160, VOLUME_THEO) respectively. These field values may also persist upon the actual conclusion of the auction call.
The field is cleared upon receipt of the new market day event, i.e. at the beginning of each new trading day (typically several hours before market open).
current value available, historical values available