oil
oil filter
oil passages
oil pressure relief spring
oil pressure sending unit
oil pump bypass valve
oil pump discharge
oil pump screen
oil return holes
oil return passages
oil ring
oscillates
out-of-round
out-of-square
outside
over advanced ignition timing
over revved engine
overhang
overhaul (v)
overhead cam engine
overheat
overlap
oversize
oversize piston ring
oversize pistons and rings
overtightened
oxygen