I think so, but am uncertain about civil ensign during 1921-35 and even 1871-1918, since increasingly after 1871, German ships used the German national Handelsflagge [civil ensign] rather than any state civil ensign.
Norman Martin, 22 Sep 2000

19th Century Flag

Reported 1820, 1848 and 1862: A red St. George cross on a blue field. At the intersection the crowned arms of the County of Oldenburg.
Norman Martin, Mar 1998

Civil Flag 1774-1871 / Landesflagge,

[Civil Flag 1774-1871 and 1921-1935 (Oldenburg, Germany)]

3:5

image by Marcus Schmöger

Reported 1848: A red St. George cross on a blue field. Landesflagge 1921-1935. In use for local and regional authorities since 1952.
Norman Martin, Mar 1998

In use 1774-1935 according to Znamierowski 1999.
Santiago Dotor, 22 Sep 2000

The St George cross and the Scandinavian cross are only different from their historic use. The first was in the beginning mainly used on land (military banner) and it was/is the symbol of a Saint, the second was mainly used at sea and was /is the symbol of Jesus Christ. Crosses in flags at sea were often set to the hoist (and not only crosses). One example: the flag of Oldenburg was blue with a red St George cross on land. At sea the flag was in the beginning blue with a red Scandinavian cross. This fact was so usual that the difference between both flags was never described.
Ralf Stelter, 13 Feb 2001

Civil Ensign until 1871 and Civil Flag 1871-1918

[Civil Ensign until 1871, Civil Flag 1871-1918 (Oldenburg, Germany)]

3:5

(

1871-1918) image by Željko Heimer

A red Scandinavian cross on a blue field. After 1871, Landesflagge until 1918.
Norman Martin, Mar 1998

Optional Civil Flag / Landesfarben

[Optional Civil Flag (Oldenburg, Germany)]

2:3 image by Jorge Candeias

Red-blue bicolour.
Norman Martin, Mar 1998

Red over blue. Norman Martin and other sources report dark blue.
Jaume Ollé, 1 Jul 1998

The Landesfarben was an optional civil flag, but when the Landesflagge was popular (as was often the case in traditionally maritime states), the use of the Landesfarben was limited.
Norman Martin, 22 Sep 2000

State Flag until 1918 / Staatsflagge

[State Flag until 1918 (Oldenburg, Germany)]

2:3

image by Jaume Ollé

A red St. George cross on a blue field. At the center a white square with the greater arms of the Grand Duchy: Central shield:

County of Oldenburg (family arms);
County of Delmenhorst (family arms);
Prince-Bishopric of Lubeck (principality of Eutin);
Principality of Birkenfield;
Lordship of Jever.

External shield:

Kingdom of Norway;
Duchy of Schleswig;
Duchy of Holstein;
Territory of Stormarn;
Territory of Dithmarschen;
Lordship of Kniphausen.

Norman Martin, Mar 1998

The Oldenburg Maritime Museum at Brake shows a GD Oldenburg flag on its site (fifth photo from top). It is a variant of the one at FOTW-ws, i.e. "State Flag until 1918" - on the Brake museum flag the small state arms are shown, listed 1-5 by Norman Martin.
The blue field, however, is dark rather than very light.
Jan Mertens, 13 Aug 2005

As far as I can say, Oldenburg flags always had dark blue, the image (by Jaume) is certainly wrong in this.
Marcus E.V. Schmöger, 14 Aug 2005

Unidentified variant

image by Jaume Ollé

I am not sure what the status of this flag may be. It looks like a variant of the Staatsflagge, or possibly an early version of the Grand Duke's flag.
Norman Martin, 21 Sep2000

Also reported is a red Scandinavian cross on blue, with (on the prime minister's standard) a white square at the crossing and arms on it. Source: States of 1897 in Arndt 1979.
Jan Kuhlmann, 5 Dec 1995

State Ensign 1893-1921

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image by Jaume Ollé

Service Flag for State Vessels and State Maritime Buildings: Like the Prussian State Ensign for Sea Services 1895-1918, except the canton contains the crowned small arms of Oldenburg. Adopted 1893 and abandoned by 1921. Illustrated in Crampton 1990 p. 42, Meyers Konversations-Lexikon 1912 vol. 4, National Geographic 1917 p. 367, nos. 999 and 1005.
Norman Martin, Mar 1998