This article in Science gives the estimated amount of molecular oxygen above the Saturn A ring:
Oxygen Ions Observed Near Saturn's A Ring.
J. H. Waite, Jr., T. E. Cravens, W.-H. Ip, W. T. Kasprzak, J. G. Luhmann, R. L. McNutt, H. B. Niemann, R. V. Yelle, I. Mueller-Wodarg, S. A. Ledvina, S. Scherer
Science, 25 February 2005: Vol. 307. no. 5713, pp. 1260 - 1262
It estimates the number of neutral O2 molecules as 10^4 to 10^5 cm^-3. However, they note it could be much higher than this range because of the limitations of the measurements. I'll take the upper number, 10^5 cm^-3. There are 10^15 cubic centimenters in a cubic kilometer so this amounts to 10^20 molecules per km^3.
The article gives the orbital velocity around Saturn at the radial distance of the A ring as in the range of 15 km/s. Actually the article explains there are magnetic effects that accelerate the various ionized molecules even faster which when exchanging momentum with the neutral molecules accelerate these faster as well. I'll use the 15 km/s number for simplicity. Then if we orbit the spacecraft in the opposite direction we would have a relative velocity with respect to these molecules in the range of 30 km/s. So if we had a scoop with a 1km x 1km opening we could collect 30 x 10^20 molecules of O2 per second. To calculate the mass of this oxygen, Avogadro's number of O2 molecules, 6.0 x 10^23, amounts to 32 grams. So 30 x 10^20 molecules is 5 x 10^-3 moles or (5 x 10^-3) x 32 grams = 160 x 10^-3 grams. This is the mass collected every second. There are 31,536,000 seconds in a year, so after a year we would have 31,536,000 x 160 x 10^-3 grams = 5,045,760 grams, or 5045.76 kilos of O2.
This article gives the density of water molecules around Enceladus:
Enceladus Eruptions Enceladus Eruptions
Larry W. Esposito Larry W. Esposito
Principal Investigator Principal Investigator
UV imaging Spectrograph UV imaging Spectrograph
On page 21 is given the density of water molecules versus altitude. The greatest density shown is about 10^7 molecules per cubic centimeter at 200 km altitude. The methane is 1.6 percent of this amount, so 1.6 x 10^5 molecules/cc. However, you couldn't move at high velocity around Enceladus such as 15 km/sec because the orbital velocity around it is so low.
One possibility would be to put it at one of the two Lagrange points that is close to the satellite. The distance from the surface would be higher than 200km so the density of the methane would be lower. You would need pumps then to draw in the methane. You would also have to separate out the more prevalent water from the methane.
Getting back to the molecular oxygen found around the rings, the researchers also found monoatomic hydrogen and monoatomic oxygen. They give the amounts of the ionized versions, but not the larger neutral amounts.
Both of these are known to be very efficient for propulsion: pure monoatomic hydrogen has about 3 to 4 times the ISP (specific impulse) as H2 with O2 as an oxidizer. A problem though that still has not been solved is how to store the monoatomic propellants stably within a rocket.
If all you wanted to do was to accelerate the rocket from Saturn then you could just use the monoatomic hydrogen as it normally combusts by bringing the single atoms together. That is, you would not need to store it. You couldn't use it though as an onboard propellant to engage in trajectory changes or landings and launches form satellites.