Samsung is rolling out Android 8.0 Oreo update to its A-series phones, on the other hand, the Galaxy S9 and S9 Plus are getting a new update with April 2018 security patch. This new update brings April security patch alongside some small improvements, bug fixes, and Device stability.
The update is rolling out as part of software version G960FXXS1ARD1 and G965FXXS1ARD1 for the Galaxy S9 and S9 Plus respectively. This update weighs at 61.33 MB. You can also download this update via the Settings > About Phone Software update > Check for updates.