How does an "Oscar II" SSGN acquire targets/launch its SS-N-19 "Shipwreck" missiles?

[ikalugin]

This depends on the tactical situation and the availability of ISR assets.

Broadly speaking:

1) you can use onboard sensors
a) such as sonar, non acoustics (range limited and situational)
b) periscope/radar/ESM (range limited and risky)

or

2) use external sensors
a) such as an ISR aircraft (Tu95 based)
b) space based sensors, such as a RORSAT
c) from another submarine, ie that got a contact via (1a)
d) land based sensors such as OTHR

option (2a) is being improved via addition of UAVs
option (2b) is not very well developed now due to the scarcity of ISR spacecraft but the work is being done on that front as well
(2c) also applies to the stationary hydrosonic arrays currently being revitilised, (2d) network is being expanded, with 2nd "container" site in the Far East.