Firstly, it'll need a couple of pumps on the pedal to pump the pistons out to meet the pads, moreso if worn pads have been refitted.
The pedal doesn't completely sort itself out until the car's done a few miles and the brakes have been heat cycled, IME.
If the fluid hasn't been changed within the last 2 years, or the fluid in the reservoir is dark coloured or has black deposits in it, it's worth changing the fluid anyway, but old fluid wouldn't be my first guess as a cause of a long pedal after work carried out, except when it occurs with a bit of heat in the system.
There are a couple of types of "long pedal". Try to identify which one you have:
1) Pedal travels down further than normal but then firms up to "normal" resistance, just lower than normal.
2) Pedal travels down further than normal and is just spongey, never really firms up.
3) Pedal "creeps" down when pressed for a few seconds, recovers after a few quick pumps.
1 is caused by pads that haven't bedded in yet following removal or replacement, can also be caused by a faulty self-adjuster (not fitted on the omega) or a failed bearing or warped disk causing the pads to "stand off". May be caused by one circuit having failed completely, but there'll be an obvious catastrophic leak then. If you consider the brakes are safe to drive on, it's worth seeing if it improves once the pads have bedded in.
2 is generally caused by a system that has air in it and needs bleeding or possibly a rubber pipe that's perished and bulging under pressure. If it happens suddenly when the brakes are hot, brake fluid contaminated with water or just plain old.
3 is generally caused by a poor seal somewhere in the system. If it's a slave cylinder or pipe connection a leak will probably be evident, although you might need to peer under the rubber dust cover to see it on a brake caliper. It can also be caused by a piston seal in the master cylinder in which case there'll be no leakage. On an Omega the master cylinder seals have been known to invert if a bleed nipple is not opened prior to winding pack the caliper pistons, giving similar symptoms.