I've never really understood ring oscillators, and I still don't.
Startup is interesting. After the power supply comes up, the stages are identical, fully symmetric, and all the nodes hang at +2 volts.
It meditates on that for about 70 ns, and then everything scatters and it soon settles down to a nice 26 MHz oscillation. The oscillation period is 5 times the sum of a stage Tpd_rise + Tpd_fall.
Version 4 SHEET 1 2324 680 WIRE 1760 -176 -256 -176 WIRE -32 -48 -64 -48 WIRE 0 -48 -32 -48 WIRE 384 -48 352 -48 WIRE 416 -48 384 -48 WIRE 800 -48 768 -48 WIRE 832 -48 800 -48 WIRE 1216 -48 1184 -48 WIRE 1248 -48 1216 -48 WIRE 1632 -48 1600 -48 WIRE 1664 -48 1632 -48 WIRE -64 0 -64 -48 WIRE 352 0 352 -48 WIRE 768 0 768 -48 WIRE 1184 0 1184 -48 WIRE 1600 0 1600 -48 WIRE -112 16 -160 16 WIRE 304 16 256 16 WIRE 720 16 672 16 WIRE 1136 16 1088 16 WIRE 1552 16 1504 16 WIRE 1968 96 1920 96 WIRE 2016 96 1968 96 WIRE -256 128 -256 -176 WIRE -160 128 -160 16 WIRE -160 128 -256 128 WIRE -64 128 -64 96 WIRE 80 128 -64 128 WIRE 128 128 80 128 WIRE 256 128 256 16 WIRE 256 128 128 128 WIRE 352 128 352 96 WIRE 496 128 352 128 WIRE 544 128 496 128 WIRE 672 128 672 16 WIRE 672 128 544 128 WIRE 768 128 768 96 WIRE 896 128 768 128 WIRE 960 128 896 128 WIRE 1088 128 1088 16 WIRE 1088 128 960 128 WIRE 1184 128 1184 96 WIRE 1328 128 1184 128 WIRE 1376 128 1328 128 WIRE 1504 128 1504 16 WIRE 1504 128 1376 128 WIRE 1600 128 1600 96 WIRE 1696 128 1600 128 WIRE 1760 128 1760 -176 WIRE 1760 128 1696 128 WIRE 1920 160 1920 96 WIRE -64 176 -64 128 WIRE 352 176 352 128 WIRE 768 176 768 128 WIRE 1184 176 1184 128 WIRE 1600 176 1600 128 WIRE 128 192 128 128 WIRE 544 192 544 128 WIRE 960 192 960 128 WIRE 1376 192 1376 128 WIRE 1760 192 1760 128 WIRE -160 256 -160 128 WIRE -112 256 -160 256 WIRE 256 256 256 128 WIRE 304 256 256 256 WIRE 672 256 672 128 WIRE 720 256 672 256 WIRE 1088 256 1088 128 WIRE 1136 256 1088 256 WIRE 1504 256 1504 128 WIRE 1552 256 1504 256 WIRE -64 304 -64 272 WIRE 128 304 128 256 WIRE 352 304 352 272 WIRE 544 304 544 256 WIRE 768 304 768 272 WIRE 960 304 960 256 WIRE 1184 304 1184 272 WIRE 1376 304 1376 256 WIRE 1600 304 1600 272 WIRE 1760 304 1760 256 WIRE 1920 304 1920 240 FLAG -64 304 0 FLAG -32 -48 V+ FLAG 352 304 0 FLAG 384 -48 V+ FLAG 768 304 0 FLAG 800 -48 V+ FLAG 1184 304 0 FLAG 1216 -48 V+ FLAG 1600 304 0 FLAG 1632 -48 V+ FLAG 1920 304 0 FLAG 1968 96 V+ FLAG 128 304 0 FLAG 544 304 0 FLAG 960 304 0 FLAG 1376 304 0 FLAG 80 128 1 FLAG 496 128 2 FLAG 896 128 3 FLAG 1328 128 4 FLAG 1696 128 5 FLAG 1760 304 0 SYMBOL nmos -112 176 R0 WINDOW 0 94 25 Left 2 WINDOW 3 72 66 Left 2 SYMATTR InstName M1 SYMATTR Value BSS123 SYMBOL pmos -112 96 M180 WINDOW 0 93 90 Left 2 WINDOW 3 78 49 Left 2 SYMATTR InstName M2 SYMATTR Value BSS84 SYMBOL nmos 304 176 R0 WINDOW 0 94 25 Left 2 WINDOW 3 72 66 Left 2 SYMATTR InstName M3 SYMATTR Value BSS123 SYMBOL pmos 304 96 M180 WINDOW 0 93 90 Left 2 WINDOW 3 78 49 Left 2 SYMATTR InstName M4 SYMATTR Value BSS84 SYMBOL nmos 720 176 R0 WINDOW 0 94 25 Left 2 WINDOW 3 72 66 Left 2 SYMATTR InstName M5 SYMATTR Value BSS123 SYMBOL pmos 720 96 M180 WINDOW 0 93 90 Left 2 WINDOW 3 78 49 Left 2 SYMATTR InstName M6 SYMATTR Value BSS84 SYMBOL nmos 1136 176 R0 WINDOW 0 94 25 Left 2 WINDOW 3 72 66 Left 2 SYMATTR InstName M7 SYMATTR Value BSS123 SYMBOL pmos 1136 96 M180 WINDOW 0 93 90 Left 2 WINDOW 3 78 49 Left 2 SYMATTR InstName M8 SYMATTR Value BSS84 SYMBOL nmos 1552 176 R0 WINDOW 0 94 25 Left 2 WINDOW 3 72 66 Left 2 SYMATTR InstName M9 SYMATTR Value BSS123 SYMBOL pmos 1552 96 M180 WINDOW 0 93 90 Left 2 WINDOW 3 78 49 Left 2 SYMATTR InstName M10 SYMATTR Value BSS84 SYMBOL voltage 1920 144 R0 WINDOW 0 68 72 Left 2 WINDOW 3 31 120 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value PULSE(0 5 100n 1n 1n) SYMBOL cap 112 192 R0 WINDOW 0 54 18 Left 2 WINDOW 3 48 56 Left 2 SYMATTR InstName C1 SYMATTR Value 10p SYMBOL cap 528 192 R0 WINDOW 0 56 15 Left 2 WINDOW 3 52 56 Left 2 SYMATTR InstName C2 SYMATTR Value 10p SYMBOL cap 944 192 R0 WINDOW 0 61 17 Left 2 WINDOW 3 50 54 Left 2 SYMATTR InstName C3 SYMATTR Value 10p SYMBOL cap 1360 192 R0 WINDOW 0 55 15 Left 2 WINDOW 3 55 55 Left 2 SYMATTR InstName C4 SYMATTR Value 10p SYMBOL cap 1744 192 R0 WINDOW 0 55 15 Left 2 WINDOW 3 55 55 Left 2 SYMATTR InstName C5 SYMATTR Value 10p TEXT 1920 -24 Left 2 !.tran 0 1u 0 10p TEXT 1904 -136 Left 2 ;RING OSCILLATOR TEXT 1912 -96 Left 2 ;JL Sep 24, 2016