How should I establish initial contact with ATC after being handed off by another controller?
In your situation:

Oakland Center, Aircraft Identification, Altitude

For example:

Oakland Center, Cessna 123AB, Level four thousand fife hundred.

See Aeronautical Information Manual, 5-3-1: ARTCC Communications.

b. ATC Frequency Change Procedures.
2. The following phraseology should be utilized by pilots for establishing contact with the designated facility:
(a) When operating in a radar environment: On initial contact, the pilot should inform the controller of the aircraft’s assigned altitude preceded by the words “level,” or “climbing to,” or “descending to,” as appropriate; and the aircraft’s present vacating altitude, if applicable.
EXAMPLE-
1. (Name) CENTER, (aircraft identification), LEVEL (altitude or flight level).
2. (Name) CENTER, (aircraft identification), LEAVING (exact altitude or flight level), CLIMBING TO OR DESCENDING TO (altitude of flight level).
NOTE-
Exact altitude or flight level means to the nearest 100 foot increment. Exact altitude or flight level reports on initial contact provide ATC with information required prior to using Mode C altitude information for separation purposes.
(b) When operating in a nonradar environment:
(1) On initial contact, the pilot should inform the controller of the aircraft’s present position, altitude and time estimate for the next reporting point.
EXAMPLE-
(Name) CENTER, (aircraft identification), (position), (altitude), ESTIMATING (reporting point) AT (time).
(2) After initial contact, when a position report will be made, the pilot should give the controller a complete position report.
EXAMPLE-
(Name) CENTER, (aircraft identification), (position), (time), (altitude), (type of flight plan), (ETA and name of next reporting point), (the name of the next succeeding reporting point), AND (remarks).
REFERENCE-
AIM, Position Reporting, Paragraph 5-3-2.