I am learning 3 part counterpoint and am doing an excersize where I am using the CF as the middle line and composing a melody above and below the CF. If you notice towards the end (chords 7 - 10) there is a part with oblique motion and a descending middle line. Basically parallel 10ths. Is this allowed when used like this and is it seen as good or bad writing?
Parallel 10ths -- that is, two voices moving together in 10ths -- are fine. Parallel 3rds and 6ths, and by extension 10ths, are even encouraged for short segments (too long -- more than about three chords in a row -- and the independence of voices is lost).
The problem here is that both voices stay on the same pitch. This is generally discouraged -- at least one of the two voices should move, and even a single voice should not repeat the same pitch more than once consecutively.
So, the move from chord 7 to chord 8 is fine. The parallel 10ths between top and bottom voices will sound quite nice. But the static top and bottom voices in chords 8, 9, and 10 are not considered good counterpoint.
(There's also a second problem: a hidden octave between the middle and bottom voices in the final two chords.)
In addition to Aaron's answer there are three other things you should maybe consider:
The first chord has no 3rd, only root and fifth. I like it but it would be considered wrong in Bach style counterpoint. It is so long since I have done strict counterpoint I am not going to comment on whether on not it is acceptable in that tradition.
In the second chord you have doubled the 3rd of a major chord if it is intended to be the dominant chord. If you could find a way to avoid this it would probably improve the counterpoint. The absence of the root, if the chord is the dominant, or fifth if the chord is a leading note chord, makes the harmony ambiguous. There again the voice leading to the submediant chord does help to compensate for any harmonic problem but to my ears the doubled leading note is not good. And if it is meant to be a leading note chord it should be in the first inversion.
The other point is that the ninth chord is the mediant chord between two root position dominant chords; within traditional harmony would be considered weak. The mediant chord in a major scale is not always easy to use. The B in the cantus firmus could almost be considered a passing note but as Aaron pointed out the static parts in these three chords is not the best solution.
Sorry Aaron, but you are wrong about the hidden parallel octaves in the last two chords, there is a good overview here:
Despite the suggestions you have made a good attempt at this exercise and it would work with a keyboard instrument. With unaccompanied voices though it would not be quite so successful. However you asked about parallel tenths so my reply is not what you asked but as you are studying I thought it may be useful.