> What is PSP process? Note:331000.1 says 
> "PSP stands for Process SPawner and this process has the job of creating and managing other Oracle processes.
> NOTE:  There is no documentation currently in the Oracle Documentation set on this process."
> But it doesn't say what those "other Oracle processes" are.

On 10gR2 and up running on Linux, background processes have an environment variable SKGP_HIDDEN_ARGS. 
On 10gR2, I believe the 7th part of the value tells us which process spawns this process. (On 11g and 12c, 
it's the 8th.) For example, checking arc0:

$ ps eww $(pgrep -f ora_arc0_$ORACLE_SID) | perl -nle 'print $& if /SKGP_HIDDEN_ARGS=[^ ]+/'
SKGP_HIDDEN_ARGS=<BG/S/x0/x13/x0/xFFF64D2A/8623/8629/x12>
$ ps -fp 8623
UID        PID  PPID  C STIME TTY          TIME CMD
oracle    8623     1  0 Nov08 ?        00:00:25 ora_psp0_oracd2

See http://yong321.freeshell.org/computer/OracleViewedFromOS.doc for more info.

Most background processes are spawned by this PSP process.

On a 12.1.0.2 server:

$ for i in $(ps -u oracle -opid=); do echo $i; ps eww $i | perl -nle 'print $& if /SKGP_HIDDEN_ARGS=[^ ]+/'; done
658
SKGP_HIDDEN_ARGS=<BG/S/W002/x0/x0/x2298/x37C61BFE/15882/15979/x9CFA/x7/x8E/x70D5D434/1412360152/1412360152>
2859
3075
SKGP_HIDDEN_ARGS=<BG/S/P008/x0/x34/x540/x37C61BFE/15882/3073/x13801/x36/x40/xA9F4AD9E/1413318621/1413318621>
3077
SKGP_HIDDEN_ARGS=<BG/S/P009/x0/x35/x5B3/x37C61BFE/15882/3073/x13802/x37/xB4/xA9F4ADA8/1413318621/1413318621>
3079
SKGP_HIDDEN_ARGS=<BG/S/P00A/x0/x36/x38E/x37C61BFE/15882/3073/x13803/x38/x8F/xA9F4ADB2/1413318621/1413318621>
3081
SKGP_HIDDEN_ARGS=<BG/S/P00B/x0/x37/xF7/x37C61BFE/15882/3073/x13804/x39/xF8/xA9F4ADC6/1413318621/1413318621>
3624
3765
SKGP_HIDDEN_ARGS=<BG/S/W000/x0/x28/x146A/x37C61BFE/15882/15979/x9D74/x2A/x66/x71039D24/1412363153/1412363153>
6964
SKGP_HIDDEN_ARGS=<BG/S/W003/x0/x29/x1353/x37C61BFE/15882/15979/x9E06/x2B/x55/x713A9A04/1412366758/1412366758>
10164
SKGP_HIDDEN_ARGS=<BG/S/W001/x0/x2A/xB9D/x37C61BFE/15882/15979/x9E9C/x2C/x9C/x71718B22/1412370359/1412370359>
13231
SKGP_HIDDEN_ARGS=<BG/S/W004/x0/x2D/x57D/x37C61BFE/15882/15979/x9F27/x2F/x7E/x71A79D8E/1412373903/1412373903>
15409
15410
15880
SKGP_HIDDEN_ARGS=<FATAL/S/PMON/x0/x1/x0/x37C61BFE/15874/15874/x0/x2/x1/x37C624B4/1411402678/1411402678>
15882
SKGP_HIDDEN_ARGS=<FATAL/S/PSP0/x0/x2/x0/x37C61BFE/15874/15874/x1/x3/x1/x37C624D2/1411402678/1411402678>
15884
...
$ ps -fp 15882
UID        PID  PPID  C STIME TTY          TIME CMD
oracle   15882     1  0 Sep22 ?        00:04:58 ora_psp0_sand12c